Alot of your problems probably stem from a mental disorder that deals with lack of confidence. I've struggled with similar sexual complications as well, not the exact same situation, but very similar which I think may have alot to do with your dilemma. It's probably nothing to worry about. If you just relax and stop thinking so much about sex you'll probably get over it. Generally relying on the problem is what causes the stress and health related issues. Although I could be wrong. It could be your partner, your body, various things, but for most young people it's nothing. Hope that helps.