That is a tough situation. That really stinks that your boss doesn't see the situation from your perspective. It must be really horrible for you to leave a job that you're that happy with and a boss you're that fond of. Is that coworker going to be there for a long time? I'm sure you've thought all the pros and cons through, though, so I won't get into any more questions. It's hard to tell what the right thing to do is, until you do it. And, really, what it's all about is what you make of it once it's done. You just have to find good in the situation when you do move and stick to that. And, I've done what you're doing several times in my life and not regretted a single time. It's all a learning experience. I wish you the best of luck.