The truth was I didn't care. I was just happy to be wanted, appreciated, useful... loved. Happy to feel like less of a failure as a human being...

She was my world. I lost the only person that gave me purpose and a sense of self-worth

think that is part your problem tbh. You are attaching yourself to someone purely because they make you feel wanted, not because they are best for you. I have been in the same position before. Its nice to feel wanted but that shouldn't be the only criteria on which to base a relationship.

many people are in relationships with people because of this. When someone better suited comes along......they leave, because they know that their current relationship isnt actually right for them.

truth be told, you have to make yourself happy, worthwhile and useful. Someone else should only expand on that. You really shouldn't be relying on someone to make you happy because that will just end in disaster.