I'm sorry to hear you are going through a difficult time. From an outside perspective it sounds like the relationship is heading into some unhealthy waters. I can only imagine the stress it must be adding to your life right now. Grief, letting go, can take a lot out of a person and then to heap on anothers issues (addiction, depression) can be overwhelming. It may be that this relationship is better spent as friends and you are being clued into that now.
This sounds like a time where you need to be around nurturing, healthy environments and not a toxic diversion from your own pain. Do you go to counseling or have a set place just for you and your needs? This has really helped me in the past and I used to swear I would never go to a 'stranger'~ a counselor, because I'm a private person. It helps to let feelings out though in a non-judgemental atmosphere. You don't have to choose the first person you call up either. Do you like walks, reading? Sometimes taking a new class to get yourself out there and in a place that shares your interests helps.
Remember you are not alone! It is okay to say no and move on from this situation. You know deep down if this is something to continue or not, trust yourself. And remember to take deep breaths...the anxiety is trying to tell you something.