I'm so sorry you are struggling! My MIL went through a similar time when her kids were in college - she had always just been with them - and it was tough. But, like the other have suggested, she began volunteer work at the hospital, joined a DAR group and took an office, and a few other things, and she never lacked for things to do.
I hope you feel better soon! Look at this as an opportunity to learn something new - go back to school - help others around you that perhaps you didn't have time to reach out to before. There is a huge world waiting for you love and support! I hope you find your new niche! Hang in there!!