With difficulty I'm afraid. Although you may choose to forgive someone, it is very difficult to forget what they have done. In this case is the issue is probably brought up whenever there is an argument or disagreement. It is not likely that a relationship will last if you are unable to draw a line in the sand and move on; this means you need to forgive AND forget in order to begin to pursue a successful relationship.
If the relationship was fairly new when the infidelity occurred, and if your partner admitted it because they felt the relationship was becoming serious, it may be worth putting the situation behind you and starting from scratch. This is easier as you can almost pretend in your own mind that the relationship wasn't yet official. In this case sit down and discuss what you both want out of the relationship and ensure that ground rules are firmly set. From this point on you will know that your partner is well aware of your expectations and is clear that you are indeed in an exclusive, serious relationship.
If the disloyalty happened during a serious relationship, I would imagine that it will be very difficult to build trust. Even if you want to forgive them for what they have done, it is always going to be at the back of your mind that they could make the same mistake again. Unless you are prepared to completely let go of the past, you may end up suffocating one another. Imagine how you will feel if your partner goes out with friends or has to go away on business; will this cause an argument or make you feel uneasy? If the answer is yes, then it is probably better for your own health and sanity (if nothing else) to quit while you're ahead.