Great question and so far very interesting answers!

I believe it depends on if its something in the past, something that cant be changed or if its an ongoing situation meaning something you are dealing with now.

If its something that cant be changed, something that has happened and its over, then accepting the fact that its in the past and you cant don anything to change it is definitely the first step. If you were involved in that situation personally and you're wondering if you could have dealt with the situation differently then in my opinion, in order to move on, first of all try to figure out if there was a lesson you could have learned from it and secondly forgive those that were involved for what they did and more importantly forgive yourself.

I think its always a good idea to learn and get as much value as you can in any situation, even when things seem to constantly go wrong.

I'll give you an example: A few years ago I woke up, ready to go to work on my motor bike. I go outside the house and realized my bike was stolen. For about a minute (not even that) I was shocked thinking "oh my god, my bike is gone"...and it suddenly hit me, I realized the simple truth which is my bike is gone, thats it, whats the point of staying in the state of negativity? Other than reporting it to the police there was nothing I could do. So I accepted it and moved on and had a wonderful day.

It all depends on how long you stay in the state of disappointment. What I've realized is that in any given situation, being in that state of disappointment never helps. The sooner you get out of that state of mind and focus on something positive the sooner you will be able to move on.