Why can't I squirt anymore?????

I've had my first squirting orgasm about 4 years ago with my ex and have enjoyed it several times more
It doesn't squirt out but it flows out during sex
With my recent bf he's the one I've always imagined being with. It's the healthiest most loving fun relationship I've had
Though we've tried to achieve that same orgasm I can't and he thinks it has to do with him since I've achieved it with others. 
I think it has to do with his large penis (6" girth 9.5" length) and the fact he lost weight and I've gained weight making me a bit insecure.
I've tried to relax put on music foreplay and make things sensual. Even the position that makes me squirt but at times my vaginal wall clenches down hard and makes it painful for him before I can achieve that full feeling
Any one who can relate???
Advice ??