This may be an opportune time to do some self-introspection and re-discover you. It sounds as if you have sold the farm on behalf of this guy as in set up your feelings and where you are in life dependent on him. From what I have seen, relationships cannot survive this way. It is is a potentially toxic situation for both of you.
If you are unable to provide support over the phone due to a phone ban at rehab, I would turn this rather confusing time for him into a period of re-discovery for you. His rehab and what he is doing has nothing to do with you. Support him to the best of your ability, but do not lose yourself in the process. When I attach to situations and extend beyond what I can healthily give someone, I end up losing myself in the end. All of these little attachments add up as a whole and I end up giving myself away and, like you, can feel lost in these difficult periods of time. It can be scary, depressing, and confusing.
Sit with yourself daily, breath deeply, and feel you. Guess who is the most important person in your life? You. Guess who you should love more than life itself? You. Guess who comes first when it comes to the entire world? You. Without a healthy and content you, there is no relationship and no healthy support you can give your boyfriend when he returns home. This is a period of discovery for him and I would be so very excited for myself and these four weeks for my own self-discovery. Turn your negative, self-defeating thoughts about being lost into positive, reassuring thoughts of finding yourself. I do not know about you, but my mind is the source of all of my problems.
You are not lost without him. If it feels this way, you may be giving too much of yourself away. From my own personal experience, it leads to great pain in the end. I had a six year toxic relationship. I hope you can mortar your foundation prior to extending a potentially toxic relationship as long as I did.
You will be OK.