Hopeful,
Welcome. How well I know that knife to the heart you have experienced. And yes, it can bring on clinical depression, related to the stress levels skyrocketing and the body's inability to return to normal chemistry. All romantic crises are stressful. But when the dark presence of addiction is involved, the shock is deep, unexpected, and a horror.
You have been with him for two years and I believe you have seen both the good man and the selfish criminal addict. And the trauma comes from shock that he is both. Believe me, when normal people break up, they do not have this horror. They will never understand what you are feeling now.
On Amazon there is a book, "When Painkillers Become Dangerous" written by Dr. Drew Pinsky and other contributors. I recommend it not only for the information you will read about pill addicts, but also because there is a chapter on Addiction as a Family Disease and this is the chapter meant for you.
You chose Hope as your name here and I would not for one minute tell you to change that. You love someone who is possessed by a compulsion which robs him of his morals, changes his personality, and controls his every thought. This is drug addiction. It does not mean that he is not lovable, nor that you were or are wrong to love him as deeply as you do.
But there is a third party in your relationship with him. It is dark and ruthless and no matter how much hope is in your heart, if you do not fully respect this force of Addiction, you will be lost. If you stay with your sweetheart without a grim resolve against enabling him, being manipulated by him, being conned by him, you will be lost. Life as you know it will dissolve beneath you as you become more and more a part of the disease.
So I hope you will pick up that book, attend Nar-Anon (or Al-Anon if Nar-Anon is not in your area--for loved ones of drug addicts) and hold tight to a counselor. Your psyche has been shattered and you need to keep yourself wrapped in a safe environment. He will bring chaos through your door right now. I would not open it for a time. Not right now.
God bless. I hope your story has a happy ending. A few do.