For me, it's most definitely more difficult to dump somebody than to be dumped. If I dump somebody, I tend to feel responsible for their feelings as well as my own, whereas if I'm dumped, I only have my own feelings to deal with, and there is no confusion as to whether it was the right thing to do or not because it's out of my control anyway.
When should you dump somebody? There are many reasons, but the most vital would be if you know that you do not love them or that you cannot picture them in your future. If you are confused about your feelings, I'd say that it's also a good indicator - when you love somebody, there is just no doubt in your mind.
PS. Liv, I'm so sorry to hear about your bad experiences. I have experienced something similar to your first relationship, and I know how terribly emotionally scarring it can be. I completely lost sight of/put up with the situation that I was in because I was trying so hard to support my ex and help him sort out his issues. Unfortunately, while he's better for it, I still have scars to heal. I truly hope that you find somebody who will treat you well, and give you the love and respect that you deserve. Relationships definitely don't need to be painful.