With stuff like losing a partner, whether to something extreme like their death or simply parting ways with the, perminantly or otherwise, one is never ready to let them go. It is like a primal urge, something that we feel deep in the oldest part of our brain, the part that also creates that fight or flight reaction, the part that craves companionship and is ever alert to potential danger. It is a very reactive part of the brain, and it takes a lot of practice to prevent it from leading us to react, to managing it in order for us to act more skillfully than not, in order to avoid making the same mistake over and over again. In active addiction, it is largely this part of our brain that drives us to you, that compulsion and urge to use despite harmful consequences of that use.
One suggestion: You may find the most success in moving on with your life, in the sense of getting yourself healthy and to a place where you feel comfortable with yourself.
In order to accomplish what you clearly want, you are going to have to learn to focus on the here and now. You are going to have to work hard at learning how to live in the present, as opposes to the past or future.
Who knows, your SO may get healthy and move back ro New York and you may have a wonderful future together. However, the reality is that it is just as likely that he will continue to struggle and either you two will drift apart. It is hust as likely that he will come back into your life as a postive, healthy influence as that he will come back into your life as negative, unhealthy influence. Yes, you have had a wonderful relationship in many ways, I am sure, as you are clearly in love. And that is wonderful, you should not deny yourself that insight. However, you are here and he is there, and you will be best of if you do not deny yourself the insight made possible when we learn to live in the present, when we learn to awaken to everyday life.
Nothing would be better for you than to learn the skillfulness of learning the ability to truly let go. To do that you must first learn to awaken in your every day life. You will have to learn how to wash the dishes in the present, mindfully, particing how to pull yourself back into the present moment of washing the dishes when your mind wanders back to your love for you SO or thoughts of the future. You will have to learn how to do the little things in the present, to wake up in lots of small ways.
The good news is that there are so many oppertunities to awaken in daily life. Actually every moment presents us with such an oppertunities, the benefit of challanging yourself to bring yourself back into the present rather than drifting off into future or past daydreams, thinking or planning
Look into Refuge Recovery and Against the Stream in NY. I am not sure if there are meetings out there, but I imagine there are. You will meet some amazing, wise people who are likewise working hard and waking up to live in the present as a means to overcome their addiction(s). Moreso in my experience than 12 Step groups, but I did have a lot if bad experiences with 12 Step groups. I just find the structure and atmosphere in Refuge and ATS meetings to be much more conducive and organized towards dealing with out practical, everyday issues, as well as recovery in general.
Good luck OP! You can do this. You have a lot of challanges and hard work ahead of you, but if you put in the effort and just try instead of giving up, trying in whatever ways work for you, big or small, and one day there is no doubt you will accomplish your goals and get where you want to in life, to a better place than you could actually imagine right now. Keep you head up. What you have ahead is almost certainly not as hard as you might imagine. And on top of that, you are also stronger than you know.