You say your friend is in love with someone else - is this their partner?

I've been in this situation where at first it felt positive. Just like you said, looking forward to seeing this person again, being happy to go to work, taking better care of myself, taking more of an interest in what I looked like when I was going to see them.....

But eventually it turned into frustration.
The difference in our situations might be that this person is your friend and I only thought the person I liked was my friend. But I feel so much better now that person is no longer in my life.