Why can't I orgasm? When it starts to get good, it all of a sudden starts to hurt

Hi, Im 20 years old and a female and just got into masterbating. I'm a virgin and have never been with anybody, but recently I got curious and started trying it. It feels good at times but I can never orgasm. I tried rubbing the clitoris and it started to feel good and my vagina started contracting and I feel like im about to orgasm but then it just dies down or stops. That even happens when i try shooting water at it with the shower head. I sat in the shower for 40 minutes once just trying, but never went over that climax. Cam someone give me any tips or tell me why that happens?

oh and also, in the shower, when it starts to get good, it all of a sudden starts to hurt (i dont know how to descride it, its not pain but its just hurts like a lot of pressure is there) and then i have to stop and pull the water away. Why is that too?