Why can't I orgasm?

My SO and I have been together for over a year now. We started having sex about 6 months ago, but I just can't seem to orgasm. I get super close, but my legs start to shake and sometimes it starts to hurt, so I make him stop. He cares about me and wants me to orgasm, and I want to know what it feels like. I'm just not sure how to fix the problem. Any suggestions? Please help.