How do you move on and trust someone when you have been hurt?

So I was in love. Well atleast I thought I was.The peson I was in love with had alot of problems at home etc. I think they sought for love in bad ways from as many people as possible to feel a sense of want and acceptance.(something they never got at home.) Well I fought for their love time and time again. But they never seemed to be able to commit to me. Yet they loved me??? So they would leave...come back...and leave again so they could still feel loved just not tied down. With no respect for my feelings. They just hurt me so many time&I put my feeling on the line multiple times. Dropping everyone I was close to just for them and then they would leave.So I finally said No not anymore.It was the hardest thing to do because I felt like I was letting them down. I promise that person I would always be there and I would wait for them But it hurt too much.&Now I just dont know how to move one. I always promised myself I wouldnt be the one who always needed to be in a relationship.Buht I was jumping from relationship to relationship everytime we did the back and forth thing.&now I find it painfull not to have someone. I just want to trust someone enought to be with and be happy.But how?