You know, krhea, I too always felt bad and guilty when I had to kick my son out...and then I realized that I had it backwards...he got HIMSELF kicked out by using, being disrespectful, or crossing our boundaries. I was simply the one that pointed him to the door. He knew the rules, he knew the difference between respect and behaving badly, he KNEW.
Each time I did, I gave him numbers for detox, numbers for rehabs and a list of local meetings. The choice was his which road to take.
It's up to you whether you follow through or not (it took me many tries), but please know that you are not a bad mother, you are not being mean...you are holding him accountable and responsible for his behaviour...that's a good thing to do.
Hugs