When you never truly get closure from something in your past, how do you deal with it and move on?

If you don't close something then you haven't dealt with it. Some things you CANNOT deal with at the time and moving on is your only option, for survival! This sucks because you postpone the inevitable. Be ready, it will raise it's head to haunt you. Strengthen your resolve to move forward. In battle it is referred to as Bypassing snipers to maintain forward momentum. Just be aware there are snipers at your rear.
PS I'm dealing with this now!