There was a time where I had problems getting hard in the bedroom area, just have a bunch of foreplay, work up to it slowly, and if it's not going in, just say "I've had a lot going on in my life right now and you're a bit stressed", so that's why.

Also see how receptive she is to sucking you off, play with her pussy put her hand on your dick, make out with her and see where it goes whilst you're both horny. If she gives you a BJ you can either try and go in, or just let her finish and this might help you relax next time you try. Sometimes it's awkward having sex with a new person even if you're both not virgins, you have problems getting it in etc.

Up to you if you want to tell her, you can always mention it after if you want. That way there'll be less pressure on you.