I have an ex that I wish I could see again, even though I'm with someone. I feel like a horrible person.
That's understandable, just because you were once together and now you're not in a relationship doesn't mean that you can't still have some feelings for her left.
I'm glad you didn't immediately condemn me, thank you. I wouldn't act on those feelings of course. But I can't deny them either.
Exactly, as long as you don't act on them. On another note, ex can be used for more than just romantic relationships. It can be used for something like "ex-guitarist".