I suppose the real question is "How do you trust again?"
In all honesty you're never going to FORGET!
Before we jump to "forgiveness" I think it's important to take a little time apart for ourselves. This may be over a weekend, week, or whatever it takes for one to sort through the range of emotions we feel after being betrayed.
Some people shoot for forgiveness immediately as a "reflex action." The shock of being cheated on and having the relationship end is too much for them to handle at one time. These are the people that will say or do anything to avoid breaking up. Several weeks later things start bubbling up inside of them. The anger and hurt is still there but now if they express it they look like a liar to the person they "forgave". A part of them is angry because in some way they feel that by forgiving they have let the other person off the hook. This is why I say it's important to take some time for yourself so you might figure out if you "really" can forgive before you promise to do so! Don't make the mistake of "forgiving" someone who is not sincerely asking to be "forgiven". If they blame you for "their actions" that's a clue they felt "justified".
The next step is to ask yourself why you want to forgive someone who has betrayed you on a deep level. Is it because "you love them very much" or because "you don't love yourself enough?" Only YOU can do the instropsective thinking to determine what your true motivations are in forgiving your mate. In all honesty. (We forgive to free oursleves) You can forgive someone and breakup with them.
If in the end you decide to forgive, you and your mate have to know it will take (time on your part) and (effort on their part) to build TRUST again.
You'll know if your mate is sincere about wanting your forgiveness by the way they act. They will voluntarily let you know where they are or plan to be, show you "extra love and appreciation" for giving them a second opportunity, share their concerns with you openly and encourage you to do the same. To forgive someone means that you will not hold what they did over their head. You both are going to learn from the past and build a better future. If however you determine that you just can't get past it then you owe it to her to let her know. Don't play the "revenge cheating game" where you take someone back just so you can hurt them the way they hurt you. Life is short. Walk away if you aren't happy being with someone.