Think of an alcoholic or an addict like a drowning person. You can try to help them, but the problem is they will pull you under. The addiction does that. You can't help someone if you aren't helping yourself first. I think if you try to understand how the addiction pulls you under, it might help you feel less guilty.
A lot of the "help" you are doing...could be and probably is keeping him from hitting his bottom so that he will want to get well. You aren't "kicking him when he's down." You are actually helping him by not helping him anymore.
If it's really stressing you out that much, it probably makes it hard for you at work. And if it's going to affect your job...well you need that job because no one is going to take care of you instead...so you have to think of yourself and your own stress and responsibilities.
Sorry if that sounded a little rambly...these are things I remind myself to keep from feeling guilty.