Why Can't I Orgasm Like My Girlfriend?

Dear Dr. Betty,
I'm 21 and I have a girlfriend at the same age who I've known for 5 years and dated her for 4 months, and it was our first time being in a same-sex relationship. (I just left my ex-boyfriend and I had orgasms all the time from woman-on-top with him.) I am very comfortable with her and trust her completely but I'm defeated most of times during our sex sessions and want to know why I feel so ashamed that it caused me to lose my orgasm ability.