Never had an orgasm...

I've read through some things here that are similar, but nothing seems to work. I've never actually had sex but nothing so far has worked.

For example, a couple days ago my guy was going down on me for maybe 5+ minutes. It felt pretty good but not amazing. After a while I got real tense and I could feel my muscles straining sort of.

Whenever I masturbate it feels pretty good but as soon as the feeling builds up I get scared or something, or maybe the feeling is too powerful and I just back away? I really don't know. Also I'm not sure if I've ever found my g-spot.

Any advice or comments would be appreciated. And yes I've tried masturbating on my stomach.