<human>: This is a throwaway and I deliberately didn't mention age. I also have to be vague with the information I give, but I'll do my best.

I met someone and we decided to start seeing each other, however it was assumed this was for a limited time because I was eventually going to move away permanently.
After the first week we started "seeing" each other I had a short trip (planned beforehand) overseas. On this trip which was a kind of once in a lifetime trip I slept with two other people. I felt like a bad person but seeing as the relationship would eventually end I continued and kept it to myself.

Fast forward to now. The relationship has gotten very serious and the way I feel about my SO now and with the feelings that have developed I would never in a million years kiss let alone sleep with anyone else. But everyday I am haunted by what happened on that trip.
Really what it comes down to is, I do not want to lose them and am more in love than I have ever been.

Deep down I know it isn't fair of me to keep this from them because they're an amazing person and who am I to deny them from finding someone better!

My heart is in my throat and I've never felt worse in my entire life.
relationshipadvice please help me, I'm so close to just telling them but if I do where do I even begin!

Condense the text into a few key points
<bot>: Cheated at the beginning of a then going nowhere relationship. Relationship turned into the real deal. Being eaten up inside with guilt