Rejection and Impending doom

I got my first rejection notice yesterday. It's from freshyarn. It pretty much says that my piece isn't what they are looking for, even though there's some "pretty terriffic writing in it". I know, I know, they send the same letter to everyone- but I'm going to pretend that someone sat down and read my story and said " This has potential!" ...Is that sad? It feels sad, but I'm not really in the right state of mind to differentiate between what is really sad and what I'm making feel that way.
I've been having a really difficult time living in the moment lately. I've been trying to not let things that may or may not happen in the future govern what I do in the present. It's just not easy. It's really hard when I keep reminding myself of how bad things could get. Rejection slips arrive, relationships get rocky and end, people leave. It happens, it all happens everyday. I'm trying to teach myself to look forward to the good moments, but lately I'm cowered waiting for the bad. How can I turn my stumbling blocks into stepping stones?