Not because they are recovering, but because they get to take the "time off" to heal and get a head start, whereas you are stuck trying to recover while also dealing with a thing called "life"?
Man, I have been here and done this. I was super resentful when my husband went to rehab in August. I referred to it as summer camp. He called me every day telling me about his perfect attendance and playing sports that he enjoys and doing art classes. Meanwhile back on the ranch I was still dealing with life like I normally had to. Oh yeah, I was a resentful son of a… I'd say it's normal for some people, I felt similarly to how you are feeling, so at least you're not alone.
How do I help myself?
I had to be real honest with my husband after about a week of hearing about his rehab activities over the phone every night. I was happy that he was getting help but holy moly, I needed him to practice being a better listener if he was going to continue to call me everyday. I was not having an easy time during his rehab stay (our youngest had an allergic reaction and went into anaphylactic shock) and more than anything *I* needed him to just listen to me vent when he called. He couldn't help me in any other way than to just listen and to not offer me advice and that's exactly what I told him I needed. And I have a rock solid babysitter so I tried to get myself to a few more al-anon meetings than usual.