That's really tough on you beckyb, I feel for you. Moments like that are hard to deal with and they creep up on you.
Enjoy what you can and try to make that your goal for the day as much as possible.
If your thoughts are getting you down gently change your focus to something good that's happening right now or earlier in the day, why was it so enjoyable, how did it make you feel? Relive those moments, distract yourself into thinking about other things. It's simple but not easy, practise will mean you spend less time reflecting on what's not there and focussing on what you do have.
- Nobody has ever learnt anything important from happiness and success; problems make us grow - Consult your plan, not your feelings - If you haven't set goals, how can you expect to achieve anything?