I was a doormat for most of my middle and high school life, and in retrospect I pity myself. It was instilled in me from a young age that I needed to be nice to everyone, no matter how badly they treated me in return. I was, and still am, a strong believer in giving second and even third chances, but as I began college and started living in the real world, I came to this realization that second chances should only be given to those who deserve your forgiveness and your respect. If someone has consistently taken advantage of you and made you unhappy, there is no reason for you to keep letting them back in. History shows that they will continue their behavior once again. At one point or another giving someone "the benefit of the doubt" becomes a way for you to enable their behavior and enable them to infringe upon our own happiness.
You need to ask yourself, why am I holding on to this person if all they do Is hurt me over and over again? You might have religious reasons, you might be childhood friends, or maybe you're just too nice. Holding on to someone who tears you down, doesn't show you respect, and takes advantage of you is not someone who should have a place in your life. The vicious cycle of them hurting you will only continue, tearing you down in the process, and using up all your mental energy to figure out what you might have done wrong to make them mad this time. Why should their happiness come at the expense of yours? No matter if you've known them for 10 years or 10 days, there's a point at which enough is enough. You need to let them go, and let them find happiness within themselves. To give the benefit of the doubt does not mean you should let them stomp all over you and then treat them like royalty in return. It means treating them with the respect they give you, and when they make a mistake, giving them a second chance, only as long as they deserve it. In the real world, employers, coworkers, and partners won't be as forgiving because the consequences have higher stakes. To better yourself, you need to surround yourself with people who treat you how you deserve to be treated so that you can do the same to others in return.