If you could answer a few questions so I can get a better understanding of what is going on and what you need that would be helpful.
Why is she wanting to break up?
What is her age and your age?
Do you live together, or have any children together?
Is she staying around for the two weeks?
What is your question...........are you looking for a way to get her to stay?
What are the issues/problems in your relationship right now?
This is a difficult situation as you can not make someone love you..........if it was that easy we all would have found happiness and lifetime love quickly.
Since she is giving you the two weeks this is a great sign that she wants to work on this. While that doesn't mean that she will change her mind it does mean that she is looking for something you used to have.
Often in these cases, it isn't changing her mind you need to do. It is accepting that you both have changed though the relationship but finding a way to still keep that spark that lead you to each other.
Lesbian relationships are no different then straight relationships.......love is love regardless of who it is with. Finding that love and learning to use it in a way that she needs is what you must do.
Forget the old standby of flowers and candy..........the real answer to your question is locked in her heart. You must communicate with her to find the real issues in your relationship. Plan a evening where you both can be together such as your home. Let her know all you want from her is a fair chance to find out what is going on. Let her know how you feel about her and how much you want and need to find the answers to the problems.
Ask her questions that will lead to her heart, such as what first attracted her to you. Start out by talking about the past when things were good. Work your way up to discussing what bothers her about your relationship now. Ask her what she was looking for when she had the affair.......do not do this in such a way that will not make her feel cornered. Let her know that is water under the bridge and you forgive her. But that you need to know why she did this.........tell her no matter what the reason your feelings are not going to change. Encourage her to be honest.........you can do so by letting her know the things that have bothered you in the relationship.
By talking to her you will find what she was looking for when she met this other person. Once you know what she is looking for you can work to give this back to her. Often what a person is looking for is that spark......that new and exciting feeling of a new relationship. This can be found easily in even a old relationship by refreshing everything. Try doing some of the things you used to do.........ask her out on a date and treat her like a first date. Do not discuss day to day things........put all the spot light on her and allow her to tell you things about her self. Lavish that attention back on her and show her how much you are attracted to her by asking to hold her hand. Maybe a walk in the moonlight or in front of a fire ask her if you can kiss her. Do not push for sex.........just kiss her. Show her you love her enough that holding her and kissing her is far more important then what happens next. Then take her lead.....once you get your footing with her try the old standbys of courting her by sending flowers, and doing little things to let her know you are thinking of her.
Two weeks is not much time but if you can find what she is lacking and if you are determined you can show her how much you love her. In the end she must decide how she feels.........you can not change how she feels. But with a little luck you can remind her how she used to feel and bring forth what used to be.