I could use some sex advice

My boyfriend and I have been dating for almost 5 months now. We’re both 18. It’s not often we have a chance to do anything physical, but when we do it’s always been good. However there’s one problem. We’ve only tried to have sex twice, but both times, I’ve had.....issues.The first time I was definitely pretty nervous which just left me totally limp. I’m a virgin so neither of us were surprised and just shrugged it off. We tried again last night and I thought it was gonna be fine. I wasn’t nervous and everything seemed fine but as soon as I opened the condom, totally limp.It’s only happened these two times so I’m not too worried about it being a big issue, but I still don’t want it to happen again. It hasn’t caused any strain on our relationship so that’s good. I’m just hoping for some advice or something to help. Thanks!