<human>: My girlfriend and I have been together for five years and I cheated on her over four years ago, six months into our relationship.

It happened at a party when I was in school, I was fully aware of what was happening when I did it, not drunkness, not drugs, and I fully know how wrong it was and that simply I was unfaithful. 

It lasted five minutes and the second it ended I felt huge shame and guilt.

 It happened very shortly after my girlfriend and I got back together, our relationship had been very rocky that month, us splitting up and getting back together but we were together and our relationship was going relatively good.

I never told her, she deserved to know then and I was too big of a coward to tell her.

Fast forward almost five years and I've remained faithful ever since and she never found out. We recently started talking marriage and suddenly that huge guilt came down on me again.

Obviously I'm not expecting sympathy, I just need to know if it's worth telling her, hurting her so long after it happened. It's unfair to her, that she would have to deal with that pain but she has every right to know.

Can you condense the text into a few key points?
<bot>: I cheated on my girlfriend sevens months into our five year relationship, never told her and remainded faithful ever since. Marriage is now being talked about and conflicted on telling her.