I feel for you... I've been involved with a woman that I love. There was no commitment however, she saying she wasn't ready for that in her life right now, that she needed to focus on college and work right now. The obvious inevitability of this is that shes eventually going to get asked out by someone else and possibly start something with them. I started stressing out, considering the possibility, trying to prepare myself... and debating whether its worth it or not... but of course being in love its unbearable to think of forcing yourself away from the person... She realized this and we talked and she kept telling me to not worry about it, that she can't commit, but nothing to worry about... she promised she'd tell me if anything happened with someone else.
well this last weekend she went out on a date, she didn't tell me because she didnt want to hurt me, normally we'd be hanging out but she asked if it was ok to have a day to "herself" to be alone and hang out in her "pajamas" all day. the day before I took her out for her birthday and could tell something was on her mind, I surprised her and got her a large gift (something I knew she wanted for a while) and took her to eat and movies (shes away from family so wanted to make her birthday special)... so she couldn't get the words out then... the next day I talked with her I was stressing out some over our situation, how I loved her but she wasn't commited to me (and admitingly I was very commited to her)... well I found out a very bad way, couldn't get ahold of her that night... so talked with a friend, he couldnt get ahold of her, she has never 'ever' not had her phone with her (she rooms with another lady so its a safety thing, she calls us sometimes when she walks to her car etc etc... so she keeps her phone with her and on at all times, even while asleep)... so we were worried, they went over and after 4 hours of worrying I heard from them that her roomate said she was on a date... after a day of stressing from the situation, then 4 hours of worrying about her that had to be the worst way to hear about that...
needless to say I'm in the same boat, The only thing I can say is that the only moments of peace I have is when I place all my importance, energy, and trust in the Lord... like purple said, put the Lord first, not your feelings for another person... that was my mistake, and am trying to rectify that... I need to just let her go, the Lord will bring her back to me if he wants that, but most of all at a time like this need to drop all 'your' wishes and desires, your what if's, your why's, and only if's... and trust in the Lord's will...
being sad is good at times, but too much is very very bad, force yourself to smile some and remember that happiness is a state of mind, its to easy to be satisfied with being sad, or even want it.