Warlocks should mostly use spell slots for non-instant spells, especially Hex, not one-time damage.
(As a general rule; everything is situational and there are always exceptions. Depending on your build, there might be more exceptions than I'm picturing. This answer describes my understanding of how warlocks are typically built around boosting the per-hit damage of Eldritch Blast to get lots of damage from one spell slot, or leave them free for utility. Or for Hexblades, boosting just your melee and ignoring cantrips is an option if you want to save your invocations for other things but still use a spell slot on Hex.)
Of course, this problem isn't limited to Warlock
But at levels 5 and above, especially 5 through 9, it becomes almost fundamentally different for Warlock's pact magic vs. normal "spellcasting" classes. They get more and more spell slots per day at higher levels, and can afford to use a few single-target damage spells per combat, like Guiding Bolt, especially if your DM doesn't throw a lot of combats at you per adventuring day.
Warlocks at higher levels (11 and higher) get more pact-magic spell slots, and some 1/rest casts of higher level spells from their mystic arcanum. Managing these is I assume somewhat like managing your high-level slots as other caster classes, but I haven't seen a high-level warlock played or thought much about it.
Your description of how you want to spend your spell slots makes me think that you're mostly looking to use them for instant-damage spells like Shatter. This is often not what you want to do unless there are a huge number of enemies you can hit with one AoE, except maybe at higher level with Fireball or Cone of Cold (depending on warlock subclass).
Warlocks are balanced around casting Eldritch Blast every round (or taking the Attack action as Hexblade / pact of the blade). They can layer enough buffs on EB to make it competitive with the sustained damage output of melee classes: 1d10 + Cha + 1d6 (hex) per beam. Unlike other cantrips that only scale the damage of their single hit at 5th, 11th, and 17th character level, higher level Eldritch Blast gives you more beams so effects that activate per attack (like hex and adding your Cha modifier) scale with level.
Compare to a Barbarian for example: at low to mid levels they're mostly just doing 1 attack per turn (or 2 at 5th level with Extra Attack), for greatsword 2d6+Str + 2(rage) damage, and have to be in melee to do it. Or less damage if they use a 1h + shield for higher AC (like you can if you're a hexblade). Some barbarian subclasses get extra damage or more attacks, of course.
Your Eldritch Blast damage normally(?) includes Hex, which costs 1 of your 2 spell slots, but lasts 1 hour if you can maintain concentration. (Or 8 hours with a 3rd or 4th level spell slot, so you can still easily have a Hex already active and 2 spell slots after a short rest once you hit 5th level.) At lower levels Hex can easily end if you're traveling or doing something other than resting for 1 hour between fights, but unless the fight looks like a pushover, or you have something else tactical / situational to do with your spell slots, Hex is a very strong use of 1 spell slot + your bonus action this turn and maybe future turns to move it to new targets. Put it on the target you want the party to burn down first, not the biggest threat.
Unlike other caster classes, you should still do respectable damage without spell slots (especially if you have Hex up, and/or Hexblade's Curse if you're a hexblade subclass, also adding your proficiency bonus to each hit. Normally save that for the main enemy of a fight, or don't use it at all if there is no one main enemy (yet / at all), especially if you expect there will be another harder fight before the next rest. Of course there are situational uses...)
As a warlock you still only get 2 spell slots until lvl 11. They increase in level up to 5th, keeping pace with the top spell slots of other caster classes.
Using a spell slot on something potentially game-changing like Hold Person is often worth it, saving the rest of the party a lot of damage and resources if it lands. Or Armor of Agathys to reflect damage to people that hit you is nice, and amazing if there are multiple small hits. Also nice if you're low on HP; gaining temp HP is as good as a heal in many cases.
You also have limited spells-known. Spending a lot of those on instant-damage spells limits your utility to the party for anything else, and puts more pressure on you to min-max how you use your spell slots for damage if that's most of what you can do with them. Fireball is very good damage return per spell slot investment. Shatter like other 2nd-level budget fireball alternatives is a lot more questionable due to the much lower damage output (3d8 vs. 8d6) and much smaller area, but perhaps still worth it at low level if nobody else in the party has good AoE damage capability but you still fight clumps of many weak enemies. You can swap it out as you level up. Shatter does have tactical uses for destroying stuff like balconies or platforms an enemy is standing on.
Buffing Eldritch Blast
It's basically essential for every combat warlock to have some way of doing good damage without a spell slot.
One of the few ways to do this that doesn't involve Eldritch Blast is Pact of the Blade so you can take the Thirsting Blade eldritch invocation at 5th level (attack twice with the Attack action, like Extra Attack for martial classes.) Plus other stuff to let you use Cha instead of Str or Dex, if you don't want to boost those stats or multi-class with something like paladin.
But otherwise your class is really built around Eldritch Blast. It does Force damage, the least-resisted damage type, so you can use it against basically everything. You only get a few cantrip slots so you probably want utility cantrips, rather than any other damage cantrips. A saving-throw cantrip like Toll the Dead is tempting for high-AC targets, but won't scale as well as EB at lvl5 and higher, and you can't add your Cha to the damage. And it's not an Attack so it can't trigger Hex. AoE or multi-target cantrips are situational and hard to use. burning down one target fast means it can't hurt anyone on its next turn, and if you have multiple weak targets you can attack in sequence with multiple beams of Eldritch blast.
The key features for Eldritch Blast:
(eldritch invocation): Agonizing Blast. Prerequisite: Eldritch Blast cantrip
When you cast Eldritch Blast, add your Charisma modifier to the damage it deals on a hit.
There are several other invocations that buff Eldritch Blast: letting it push or pull 10 ft (once / turn), giving it 300ft range, making it reduce the target's movement speed.
Casting Time: 1 bonus action
You place a curse on a creature that you can see within range. Until the spell ends, you deal an extra 1d6 necrotic damage to the target whenever you hit it with an attack. [...]
The eldritch invocation Agonizing Blast is also essentially mandatory for good sustained warlock damage output, unless you're a melee warlock and you only want a cantrip as a fallback while closing to melee range. The flexibility to shoot from range is really powerful, though, even if you do slightly more damage in melee (e.g. with a 2h weapon, or a 1d8+1 magic weapon.)
Doing solid Eldritch Blast damage means you're always a threat even without your spell slots. And it means you can save them for tactical usage, like Thunder Step to rescue an ally from the front line, or for control. Or just for Hex for more damage.
If you feel like you need your spell slots to do useful damage, make sure you're doing 1d10 + Cha damage with your Eldritch Blasts even without Hex. That will take a big jump at 5th level when you get 2 beams per cast.