Why do I always blame myself for things I didn't do in the relationship?

because u blindly love the other person.we tend to blame ourselves when we like the other person and place them in front of us.

Because you feel guilty about the past, but it is not your fault, as you could not have foreseen what would happen.

Usually because the other person in the relationship would have a controlling nature. With a controlling nature it would establish you to be in the wrong most of the time.

Unfortunately sometimes our other half will guilt us into feeling bad so that they don't have to....

Because Im insecure and am ready to step down many a pegs and for that I prepare myself by begining to blame myself in order to keep the relationship going

Because you do not see how great it a person you are and maybe the other person was making you feel as if you were the one that was going something wrong

It most likely is in your nature to blame yourself - after all it's the easiest thing to do. But you need to understand that some things are out of your control, and you just have accept it when other things go wrong. Blaming yourself will not improve your situation, it's okay for other's to take responsibility for their actions.

When I did that, I did it because I thought that the other person was the world to me and I just wanted them to stay.. Mainly because of guilt and wanting. (wanting them to stay)

You feel that you are to blame completely and that's not true, always remember a relationship is 50/50. You're trying to make things better so you think you were wrong completely, but you aren't.. And it's okay if things don't work out immediately, over time you'll realize it's not just you, just hang in there.

Some people can't take responsibility for their actions no matter what points are thrown at them and some people do the opposite and blame themselves for things they had no control over. You just need to learn that you can't change everything and some things are just beyond your reach.