Firstly, let me congratulate you on being able to get over someone you were previously in a relationship with. You clearly believed it could work, and no doubt invested a lot of time in the relationship, so its failure is therefore making you feel hurt and bitter. This is a normal reaction. You must remember not to blame yourself though, because for whatever reason your ex has decided to move on, and although it hurts, this is the only option for you. Everyone has been in that position where we secretly believe we'll get back with someone even though it has been explained otherwise. I think it's the process of rejection which tears us apart, because that in itself feels like a failure and can only be taken personally. Yet, despite knocks like this, you have to find a way of re-gaining your self-confidence and getting back up and starting again. You need to use the support of family and friends to help you feel better about yourself, start new hobbies or carry on existing ones to forget about the negative thoughts.
I think it took you so long to get over the split because to begin with you had low confidence and self-esteem which she was probably aware of. Therefore, you reacted badly and couldn't get up again. Added to that, you say you suffer from depression and so things seemed unbearable beforehand, and this hasn't helped.
I think it would be wise to make an appointment with your doctor explaining your physical symptoms (lethargy which is actually part of depression), and that you feel terrible mentally.
Good luck.
Feel free to PM me.