You are going to have to deal with your insecurity or break it off completely.
She made a mistake, but at least she didn't sleep with him and she fessed up to you on what happened almost immediately.
Date around and make sure that you really want to get married, but if you do decide to marry her, don't bring up the kissing/cuddling again or your marriage will be doomed.
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One must be fond of people and trust them if one is not to make a mess of life. - E. M. Forster
You may be deceived if you trust too much, but you will live in torment if you don't trust enough. - Frank Crane