perhaps memories of a bad event are haunting you.
just remember that the present day is not to be ruined by memories of the past and that things are better now. Anytime you feel sad, remember that there will always be good reasons to be happy. Anytime i feel down, i treat myself to something i like, or just a relaxing day. Works wonders.
You've already come so far. I know that you can overcome these negative feelings. If you need someone to rant out to, i'm here too!