Guilty Conscience

I’ve been dating this guy for a couple months and it’s been good. I’ve known him for about 8 years (we went to high school together) and just decided to finally take things to the next level. This past weekend I went to my friends hometown for his brothers birthday and ended up sleeping with my friends cousin. I’m not going to blame it on the alcohol or anything like that because I knew exactly what I was doing. My heart is heavy and I hate what I did because I know I wouldn’t want him sleeping with other people. I’m hesitant in telling him because I know what we have won’t be the same & I’m going to lose one of the most important people to me. Any opinions on what I should do?