Whats wrong with my erection!

Hi, i am 18 years old and i was just seeing if anyone could help me out me and my girifriend have been togther for about 2 years now and the first few times we have had sex but than all of a sudden i cant stay hard when we are about to have sex again, i will be hard when i put the condom etc.than when i go inside of her i just wont stay hard whats going on here?

my boyfriend and i have been dating for about a week, i've know him for a month. we have been doing alot of fun things doing sex, and when we were going to get ready for sex well... he got hard then lost it. he says is lack of stress, i don't know what stess he has. if anyone can help me help him. also when he does get an erection well.. it will get soft inside of me, and then his balls will hurt. will that be because he did not ejaculate?.
someone help!!!!