I was playing a moon druid and cast the polymorph spell on myself, changing me into a T-Rex, which caused my Int to become 2 and my Wis 12. I realized my targets had immunity to non-magical damage which is all a T-Rex can do, so I wanted to stop concentrating. In-game, we had a character with the capability to speak to me in T-Rex form and successfully explained to me that as I am now I was useless and to stop the spell.
I made the check to determine that my current form could not affect the creatures, but when I stated that I would stop concentrating, the DM said that, as a T-Rex, my Int was too low for me to realize that I had a spell I was concentrating on that was keeping me in this form. So, I was stuck as T-Rex until the HP was depleted or the spell ended.
Was my DM correct or can a polymorphed creature still choose to end concentration?