female orgasm?

how do you know when you're going to cum? I honestly don't know how. I've been with my boyfriend for 3 years, he's been trying so hard. I just can't. I've had about 4 partners and nothing happened with them. I was able to by myself but that was the first and last time. I've googled that it feels like pee but I can't just let it go. if I have to pee, we stop and I pee then come back. I told him let's try in the shower so I won't be so scared and I just can't release it in there. my body still holds it well, a few nights ago I felt it again when I was riding him and I stopped and the pee feeling would go away. I finally let it go and started crying because I was afraid I peed myself and I did. he kept saying it was okay but I was so embarrassed and felt nasty . some was pee and some wasnt because it didn't smell like pee. but I wanna know how do you know the difference?

I'm 20, he's 21, we have 2 kids 1 & 7 weeks.