No, you are not unbeatable, in fact, you are exceptionally beatable.
Once my character sees you have a pole-arm, a dead give away in how I am going to approach anyone holding this type of weapon. A regular attacker falls prey to this type of weapon, so to avoid getting hit by it, I just declare I am using the Disengage Action.
If you take the Disengage action, your movement doesn't provoke opportunity attacks for the rest of the turn.
and since I no longer provoke OA's, I run up to your face, then casts a quickened spell, casts a Bonus Action spell like Hex or any number of other Bonus Action choices. Your character can't do squat.
"But wait! Sentinel allows me to hit creatures who are using the Disengage action, so I still get the OA!"
Unfortunately, Sentinel doesn't work that way, it is very specific about when this special ability applies. You can only hit a disengaged character that is leaving your 5' range, it does not apply to entering (and according to Jeremy Crawford @ Sage Advice, the 5 foot specific is your reach, not your 10 foot polearm reach).
Creatures within 5 feet of you provoke opportunity attacks from you, even if they take the Disengage action, before leaving your reach.
Nor does your 10' reach apply to Sentinel, as it only applies to those within 5' (see above). Disengage does not stipulate which way you run, nor is it negated by actions taken with the Bonus Action (again, see above). The Rogue can use Cunning Action to use Disengage as a Bonus Action and then full attack you, and none of your planning will work, and if you take 5', 10' or 15' to move away without using up your action taking the Disengage action (Bonus Action if you have levels in Rogue), then THEY get an Opportunity Attack on you instead! The Swashbuckler Fancy Footwork completely destroys the entire Sentinel feat, so long as they make 1 melee attack at you. Not a hit, just a "melee attack", so, BA to Disengage, Walk up to you and use full action attack, probably with sneak attack, and none of the Sentinel abilities apply.
During your turn, if you make a melee attack against a creature, that creature can't make opportunity attacks against you for the rest of your turn.
The other way this specific kind of character dies quickly, is by a range character. I know a bowman in one game I play who would make quick work of the type of character who is using his polearm to attack multiple targets, and my Sorlock would have a blast with his agonizing, repelling Eldritch Blast, knocking this type of character back into walls for additional "falling damage", or better yet, off a cliff and watch him drop, "Bu-bye."
I appreciate that you have asked the question, because so many players think they have figured out some great "system", only to find that they didn't read the rules properly and this "unbeatable character" they thought they had, is now getting his butt handed to him repeatedly, because he doesn't do what they thought he did.