Welcome to the forum, gg. You wrote a very honest post. And I'm sure you must feel lonely for him, which is normal. We miss people we love when they are away from us.
In recovery programs for addicts as well as for codependents, the advice always is to take "the long view." This means do not expect anytime soon to stop hurting, grieving, being mixed up, feeling lost, and feeling anxious when there is a change in the relationship, whether that relationship is with a drug or with a person addicted to a drug.
Addiction is a battle, it is a long battle, and unless a person knows how to stay in the present, and to try to do the most positive thing she can in the present, the partner of any addict will lose herself. It is better to expect a great deal of emotional discomfort rather than have any hopes or wishes that things will be happy and comfortable any time soon. Relationships with addicts are very hard.
Anyone who tries to talk you into walking away from him at this time will likely be wasting time, so the better advice, I think, is to tell you that if you want to be in a relationship with a drug addict, you need to find a counselor as well as a 12 step group for partners of addicts, a group like Nar-Anon or Al-Anon. Find a group with enough younger members there so you can feel understood. If you try to handle this crisis in your life alone, we all here know that you will lose your way. It is best for any family member or romantic partner of an addict to enter her own form of counseling and recovery, the sooner the better.
You don't have to cage in your emotions. You feel what you feel. But your thinking: that is something you have some choices about.
There are many good articles about relationships with addicts on the blog posted by Cynical One. (blue box at top of the page for blogs). Reading those is a good place to start for now. That will help with your thinking.
I will say that people in active addiction mostly destroy every person in their lives, emotionally. They leave a path of emotional devastation, and you are at risk of experiencing that. So I want to give you a heads up: if it happens, it is part of the syndrome of addiction which has impacted you. It is what happens. Do not blame yourself. Your first impulse, when he starts hurting you, is to blame yourself and try to "be better and do better." This leads to nothing but an emotional breakdown. If you are with a counselor or a group, you will be better protected from that.
More members will have some experiences for you, gg. Again, welcome to our forum.