"I wish I could believe him when he says he wants to stay sober. "
It may help you to realize that oftentimes addicts DO believe and ARE very sincere about what they say AT THE TIME they promise or declare they want sobriety (or anything else we want to hear). The problem is, addiction doesn't allow them to keep those promises and we have to simply realize that the addict is not able to bring those promises to fruition.
Remembering that will allow you not to put much stock into the words in the first place and avoid the disappointment in the second place (when whatever is promised doesn't happen).
LH