I had to reply because your message reminded me so much of a time in my life. When I was engaged to be married (13 years ago!), I went through the exact same thing you're going through. I was so afraid of my happiness, that it actually scared me and I was so afraid of losing it all. I didn't want to leave home because I was afraid of never being able to come "home".
I took the leap of faith, though, and found that my life was wonderful as a newly married adult, too. I still have episodes of anxiety, which usually occur when big changes happen in my life (the birth of a child, moving, new jobs, etc), but I have learned that it is just part of who I am and it will be okay if I just forge ahead.
I have had some counseling along the way when I really start to feel out of control and I did take paxil for several months when we were preparing to go to russia to adopt our 2 youngest children, which helped me enormously at that time, but overall, i've just learned that this is who I am and it's okay and i'll get through it if I just don't stop moving.
Take the leap of faith and move to your new place. You'll probably find that your life is richer and better than you even imagined. And do whatever you need to do to not think about everything so much. I'm a true "overthinker" and have had to learn that thinking everything to death does nothing but hurt my presence of mind.
I'm wishing you the best. You'll get through this. It will pass. Hope this was helpful.