You sound like you've got a lot on at the moment with friend and family troubles, and having recently split up with your exgf too. If I were you I'd be inclined to wait and see how it goes with this other girl, don't rush into anything.
I was in a relationship which ended in a similar way to yours. My bf's feelings changed towards me, things went sour, but when we actually split I still felt a lot for him. I'm sure he was probably ready to move on and go out with someone else not long after breaking off with me, but if he had got a new gf and I'd found out about it, it would have been like a kick in the gut for me, even though I knew there was no hope of getting back together with him.
You don't say how recently you split up, but if it's less than a month then I'd be inclined to give it a bit more time, for your ex's sake aswell as your own.