Why is it so hard to let go of people that you know shouldnt be in your life?

My boyfriend and I broke up 2 months ago and I was doing good with getting over him and then one day he brought me 2 doz roses and professed his love for me and told me that he was so sorry for everything he did wrong to me and that he would do anything for me like go to church with me ( which is a big deal to me and he knows that ) and i cant decide whether or not to take him back. Im not really sure what to do. like i dont want to be with him but at the same time i do want to be with him. im really confused and he wants to get married but im not so sure i do. any advice? thanks a bunch!