Im stressing out! Flat head.

My baby has a flat head. The doctor told when he was a baby it would correct itself with more tummy time. I swear this kid stayed on his tummy. Now, I still notice that it’s still kinda flat and it too late for a helmet. Im so upset I was not more adamant about getting a helmet. She kept saying they don’t do much. I think I messed up his head for good. Anyone on the same boat or had a kid that it corrected itself as time went on. My doctor keeps saying it will get better, but it doesn’t and he is a stomach sleeper now. I’m so worried I wanna cry. I feel so stupid!