If someone is causing you emotional stress, and you've tried your best to rectify the situation, then I think the next step is to try create some distance/emotionally detach from that person. That doesn't mean you can't be friends with that person, but it does mean taking a step back and reevaluating the relationship.
I know, sometimes things are so hard that we become absorbed in our problems. I think everyone goes through that. But then I think about all the people in the world who are going through a LOT worse, and that puts things into perspective. That's not to say what you're going through isn't difficult, but it could be worse, right? There are so many things to be grateful for, think about those things. Try to think more positively about yourself. I wonder if it would be worth seeing your therapist more often, because you describe the situation as getting pretty bad. Do you find that you feel better after seeing your therapist?
Friends like that are so precious, many people wish they could have such friends. You're very fortunate! It sounds like you're doing it right to me! I think communication and mutual empathy/understanding are important in any relationship, so it's great that you're working on that. At the same time, try not to rely on your friends TOO much. If you're feeling bad, your initial reaction shouldn't be to reach out to a friend. I think it's important you try to deal with difficulties on your own as much as possible. You can learn to be your own therapist! But of course, sometimes we need to reach out to friends, and there's nothing wrong with that.
There's always some explanation as to why we're in the situation we're in. It's often unfair, and heartbreaking. Yes, it does suck. But, as you know we can't change our past, and we can't go back in time... which is exactly why you're TryingToMoveForward, right?!

But with hard work, you can change the future. How has your therapist suggested you go about dealing with these issues from the past?