Wanting to be happy is your birth right. I think you know deep down that that it wont happen until you let go of your ex. You know what it's like being with him, as you've given him 3 years of your life... and as long as he is an addict, things wont change for the better. I'm sure you guys have shared good times, but the bad outweighs the good and there is just too much pain that comes with being with an addict.
Let him go, heal yourself, and find your happiness. I can assure you that happiness is not him. It comes from you but you have to give yourself a chance. When the memories of him haunts you, just quietly whisper to yourself, "I wish you the best, but I have to let you go." I find that very therapeutic and helpful in my own path of healing.