@mollie1111,
Hi Mollie.
I do understand. I also think honestly, you both are young. You are meant to experience life, enjoy life.. Marriage, being with one person for the rest of your life, faithfully is a dang long time if you start at 17, or 20.. Alot of people get Divorced because they felt they missed their youth.
Your feelings maybe hurt but if he was faithful before, and now and always within a relationship, you are doing mighty good. Imagine all those girls that are cheated on whilst "in" a relationship? I think you should always look at the positives not the negatives. Negatives make you have doubts and fabricate things, they also create such a drift that no relationship can survive. Ever. And, constantly at him over it, reminding him eventually will push him away.
Think of the positives, hanging out with you again made him realise that he missed you and didn't want to be with someone else.
Youth is hard to get through but after that, pftttt piece of cake....