My comedowns are very predictable:
Day 1 - so sleepy!
Day 2 - crying at TV adverts.
Day 3 - Massive anxiety and self doubt
Day 4 - Bad mood, likely to rage about anything!
Day 5 - Pretty much ok.
Personally I haven't found a way to fix it, BUT, when I feel this way, I take a second to remember that it isn't real, this isn't how I would normally feel. Take a moment, breathe, do I need to react this way? The best way to deal with a comedown is learning to recognise the patterns in your thoughts and try and cut them off. Probably a bit like how Cognitive Behavioural Therapy works.