First, talk to your primary doctor. Second, I think most people are "co-dependant". My opinion is that it's just a $10,000 word that therapist starting using years ago. I would like to ask if you exercise? Have you done any research? You may find you have a chemical imbalance. Lack of serotonin can cause our moods to go down. There are several things you can do (without medication) that can increase serotonin levels - something as simple as chewing a piece of gum or sunshine. What do you do for yourself? You asked where do you start - well, repeat the following: "IT ALL BEGINS WITH ME". Staying positive is number 1 even if you have to keep starting over. Try to take a walk, anything. As hard as it is and as much as you may procrastinate, physical activity will improve your mental state. Test me on this, you will find it works. And do at leaste one thing nice for yourself - you are worth it.
I agree with lonelyinacrowd. I do a lot of walking, and there are times I just don't have the motivation to do it, but feel so much better afterwards! I am using the "rubber band" therapy and it is working wonders for me. You keep a rubber band on your wrist, and when you start to feel sad, angry, worried, anxious, etc, just snap it. It will sting but after just one week of this, I find I'm not having as many negative thoughts. I didn't have to snap it one time yesterday, and didn't even realize it until at the end of the day while thinking what a great day it had been. If you want to read a good "self help" book, try Deadly Emotions" by Don Colbert, M.D..