Been feeling the same way.
Met an old friend this month and we got talking about rest of our friends. I told her how I was feeling.
She stopped me midway and told me,
"You would have been there too, had you made that choice (in a job, doing research etc.).But, this is the choice you have made , you shouldn't do it half heartedly."
Things have improved a little from that point onwards. I have also kept myself busy with books. I guess that helps.