Her sleeping with someone else should be the number one indicator that she is not committed. If you want to get married you should have monogamy and mutual respect. One solution which she may or may not be open to is couples counseling. Then you can find out why she cannot commitment. There is a big difference between not wanting to commit and not committing to you. I from what you said wonder if it is the first one. Unfortunately you can't get a commitment phobe to commit until they want to. You can be the greatest guy and she is still scared. Try to sit down and work out a plan that works for both of you. Something reasonable that she can live with. If she won't then you have some important decisions to make.
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