Sorry for what brings you here Shellart. There really is not a good answer as to "why" addicts do what they do, other than that they are simply addicts. And addiction is a horrendously selfish affliction.
The best thing you can do is take care of yourself...you'll find a lot of support here on SR. Check out our friends and family forums too if you like, you will find others who have been in similar situations and know how to help.