I'm thinking about posting a confession about my crush on a man that's already in a relationship.
Why do you want to confess to having a crush on a man in a relationship?
I am just tired of holding it in and want to let it out.
When you have a crush on someone it can be hard to keep it to yourself, but if that person is in a relationship then you need to keep your feelings private. If you pursue someone who is already in a relationship, then you will hurt that person's partner and you may be socially stigmatized as a home wrecker.
I don't care about any of that, I just want him.
Well you can tell the person how you feel, however acting on the urge is still a bad idea. Just let the person know, and maybe one day if they end up being single you can try and form a relationship with that person.