Slept with someone else...

So, my SO is my first love and the person I lost my virginity to. The sex was amazing, but over time things have become less.. “exciting.” We did get engaged, broke up (I wasn’t the one who ended it) we split up for 4 months and in that time I started seeing someone else. Z, as we will call this other person, was from another state and came down for a week and the sex was absolutely mind blowing. Z & I didn’t work out, my SO and I got back together... and now the sex doesn’t feel as good as it did before. I have no idea what to do and I definitely don’t want to bring this up 😬😬