Orgasming is hard af

I have a gf who does everything right when we have sex, lots of foreplay and clit stimulation, but I still can never cum. I've made her cum before, but I can never get there. Sometimes I think I'm getting close with penetration, but it feels really weird in the bottom of my stomach, my lower abdominal muscles clench painfully even if I try to be really relaxed. It also feels like she's touching my intestines even though I know that's not possible. When she goes down on me, sometimes I also feel really close, but then it all feels like too much. I always feel tired during those two moments and want to take a break. Does anyone have any advice on how to alleviate that weird feeling or be able to orgasm?