I have been in a relationship for 2 years. My girlfriend loves me, but I got to know that earlier she had a physical relationship with someone else. She accepts it, and says that she loves me, but I am having difficulty to accept this. What should I do now?

If this physical relationship was before you met, you forget it as none of your business. You move forward and accept it as she has.

Don't be disappointed because you were not the first, or only. Be glad that you are the NOW, and relish in the glory of it, and enjoy it.

If you are going to allow this to ruin a good thing then you should let her go until you are mature enough to put things in there proper perspective.

Is she YOUR first?

And in case she is not your first, or, she had the physical relationship while she was dating you pretty much says about her commitment level. Its pretty easy to figure out how much she cared about you while making that decision (even if it was on impulsion)

So are you saying that she was seeing someone else WHILE you two were dating? If so... if I were you, I would not accept that whatsoever.