Why is it so hard to move on?

It depends on the situation and each person involved. I know people that are married and have a family and property together. All of their families functions would be split if there was a separation. Property is also hard to split. People settle or do what they think is the most important. Some people value stability and familiarity over a split just because they are not all attracted to their significant other anymore. People get old. It's life. I'm not saying it's right. I'm just saying it is life and it's not always feasible to think one will find a new "mate".

Sometimes it's nothing more than what's familiar. Change can be scary.

It hurts to walk away from someone you love even if they are toxic

Loss of self esteem will be a contributing factor

If you have spent so much time in somebody and invested so much in a relationship, given all your love and passion, then that person will most likely have a small place in your memory and your heart. It may be hard to move on because you are holding on to what you once had.

If they love you they will fix the bad as best as they can

O,that's common: cognitive dissonance!

it was very tough for me. but i had to do it for my good

Some people have taken the position that even the wrong "something" is better than nothing.

poison isn't always easy to accept