Why can't I orgasm? ?

I'll run my clittourous and I'll have a tingly feeling and then the pleasure suddenly turns into too much and it's not pleasant anymore but I wanna stop. If my bf is giving my head I sqirm to get away and if he continues it start to just be more and more uncomfortable--- we don't concentrate right on the "bulb" (idk the proper name) but it happens every time w/o fail --- my vag will also papillate like I want to *** right before this happens too but it never comes out -- I want an orgasm :( please help!? What am k doing wrong?