Brutal honesty time - you have already cheated on your partner emotionally. I also reallllly dislike when people say they're "worried" they might cheat on their partner as though they have no control over the situation. I'm sure you've heard plenty of horror stories about dating a coworker, but consider if your coworker is only interesting to you because you're hitting a slump in a long relationship. If you don't want to be with your partner (which you clearly do not), just end the relationship. Would you rather lead someone on or live life as you want to live it? I know that living together complicates things; see if there's a couple friends who have a couch for you to crash on while you look for a new place and just break up with your partner.