Make sure to take your time with foreplay, and as you say, lube is a good help. Work your way up slowly and don't feel like you have to rush anything, and as others have suggested, go get tested for your own peace of mind.
The fact that it's been hurting will cause you to expect pain, which won't help if you're tensing up, so try to relax and remember that there are a lot of other fun things you can do - don't feel like you're disappointing your bloke by wanting to take time.