Orgasm complications

I haven't been sexually active for long but I just haven't experienced an orgasm. You know the big "O" I can have a mini orgasm by pleasing myself but when it comes to sex it doesn't seem to happen even with clitoral stimulation it isn't as powerful. When I'm receiving oral sex I can feel myself getting there but it's like I get so sensitive that I refrain from letting myself go because I feel like I might lose it....but I get tired afterwards like I did orgasm. Idk any advice?