I agree with Janissa.  Assuming he knows about your past fling and he is still with you, means he still loves you and is willing to work it out.  Hopefully you told him about the affair and he didn't find out some other way which shows some trust is still there. Absolutely do not keep dwelling on the past and mentioning it to him.  He's moving ahead three steps and every time you mention it, you kick him back two which will never allow either of you to move on with your relationship. 

Treat is as any special Valentines should be.  Don't try too hard though because it may just backfire on you and push him away. 

Move ahead with your lives if that's what you both want and try and forget about the past and focus on your future.   I also hope that you are sincere when you say it won't happen again and you're truly sorry, because he deserves the truth.

Best of luck to both of you.