Healthy relationship + bad foundation... Please help

Hey there..

I'll try to make this as short as possible. This girl and I have liked each other for two years, and have been building and nurturing a good friendship at a distance (seeing each other a few times a year at events). 3 months ago I received a bad concussion causing all sorts of mental and nervus system issues - being overly emotional is one of these. In the midst of that she asked if we could slow things down a bit. I outwardly, and mentally completely agreed with her, but emotionally felt really hurt, so I started seeing someone in the city I'm now in...

This new person is great, she's so kind, and is good for me. We communicate well and connect on a lot of levels. But our relationship has moved quite fast and we are practically moved in together. During this whole time I've pretended like I've gotten over the other girl of two years... Which I haven't. And now I can't stop thinking about her, and it's the same for her... I feel awful being in this relationship both because of how I still feel for this other girl, and because this relationship is pretty healthy (minus the obvious)...

I feel like I need to end this relationship, but I feel ill thinking about how much of a shock this'll be for her, when she sees me as a potential life partner, and that all her friends and family like me, and vise versa, and tons of her stuff is in my place...

Please advise me, it's keeping me up for hours every single night, and causing so much anxiety...