How do you deal with insecurity issues after you've been cheated on by an ex?

I'm curious to know how anyone can settle issues of trust once it's badly broken by a girlfriend / boyfriend. I know that it affects so many people in their future relationships and can ruin them just because of their bad past experiences. Is it possible to let go of your past completely without letting it take a level of control over your partners actions.. because surely your bad experiences stay with you forever and you can't just forget them when you encounter a situation that makes you feel uncomfortable.