No, you are not unbeatable, in fact, you are exceptionally beatable.
First, there are range characters, like archers, and spellcasters who don't care about melee feats, and can "kill you lots".
However, once an intelligent/experienced character sees you have a pole-arm, it is a dead give away in how to approach anyone wielding this weapon. The easiest way to attack is just declare 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, Any character can run up to your face, then casts a quickened spell, casts a Bonus Action spell, make a bonus action attack, or any number of other Bonus Action choices. In fact, a Rogue can use their Bonus Action to use Disengage, run up and use their entire attack action on you, and 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, and confirmed with Sage Advice). Disengage does not stipulate which way you run, nor is it negated by actions taken with the Bonus Action (again, see above). If you decide to move away, you either not attack and use the disengage yourself (no Opportunity, and now no regular attacks), or you choose to attack normally and move away, now THEY get an Opportunity Attack on you instead! The Swashbuckler Fancy Footwork completely destroys this entire combo and they can run around in front of you all day. Note that it is just an attack, no actual "successful hit" is required.
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.
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.