Big O – help?!

Hi! So i’ve been with my boyfriend for a year, and even though sex is always great I have only came once and i’ve never had a big orgasm (even though i’m pretty sure i’ve been close). I’m 21 and getting very frustrated! I have a low sex drive and always struggle to get in the mood. He always tries so hard and i always feel bad because he just wants me to feel good too. Any advice?