[MysteriousPickle]

Dude. STOP stressing. You're stressing about the sex you planned to have your partner, therefore your penis is all, "stop, I'm scared!"

Lol. But seriously, stop stressing, that is the beginning of your problems. My boyfriend had the exact same problem... He would get so stressed out about not being enough during sex or whatever, he'd get nervous that we were going "to do it" and wouldn't be able to keep it hard. There's nothing wrong with you, you're just stressing out too much about the situation. Then you stress about the effects of your previous stress, and on it goes. Just relax. Good luck.

It took my boyfriend a while to finally ejaculate when he first started masturbation. Is it because your hand gets sore, or does your mind wander and you forget what you're trying to achieve? Maybe try watching some porn while you masturbate, this might help. He too used soft things like a blanket, or mattress etc.
About your sensitivity... Don't worry about it. My boyfriend is still super super sensitive and we've been sexually active for over a year. It's best to not desensitize it, as it will bring you more pleasure during sexual activity if it's more sensitive.