My SO may be kinda rubbish in bed...

So my girlfriend and I have been together over 8 months now. We are totally and completely in love but she's just sort of lacking in the sex department. I've never actually had an orgasm with her. She's the first and only person I've ever slept with but I can orgasm just fine on my own. Some days it's really great sex in all honestly but I still don't quite make it. I know it could have just a lot to do with me but I also want to know the best way to like ask her or advise her on....how to be better I guess?