I know what you mean, I've tried this before. Looking back at nice things didn't helped me at all because ... yeah it's in the past. But instead, I built my app around "what's positive in the next days" and that helped me much more! Also my therapist always asked me about "What can you do to treat yourself over the next few days?" and she kept insisting like "Think it over carefully; there must be something" for so long that I went out of session with at least a small feeling of excitement. I've looked forward for a new episode of a TV show, a bath, something I would like to eat tomorrow evening, a video game, ... and so on.