Have you ever had a crush on someone when you're already in a relationship?

If you haven't, just know that it's an awful feeling.

I'm not talking about just like... Looking at someone like "oh youre hot oh wait i have a boyfriend" nope. This is someone from online that I've been talking to recently that I just can't get out of my head. ;~;

And like... Dylan would be so jealous/upset that I even HAVE an online guy friend that I don't know what to say to him.
He's very set on "forever" with me. But I just graduated high school. And while the thought of spending forever with someone is amazing, I'm just not sold. I don't want to hurt him because there are still feelings there, and I just... I don't know what to do.

I don't want to stop talking to this online friend, either, because he's helped me get some bottled up feelings out and I just really appreciate that... Plus he's fun to talk to and I feel like I can be myself whenever he's online.

Has anyone here been in a similar situation? If so, I could really use your help.