Ugh! I understand what your going through, and I think most of us have all slipped throughout our many attempts of quitting. It is going to be even harder for you having a loved one living with you that smokes. BUT...don't let anyone say it can't be done, because YOU can do it!
I think if you continue to be discoraged with yourself because of that one lousy smoke, then you will make it harder on yourself to stay a non-smoker. Just forget that stupid ciggarette, and keep telling yourself how crappy it tased. So what if you slipped, get back on, and don't look back girl!!
A tip that I have been doing when my boyfriend wants to light up...I go for a walk. Sounds simple enough, and it is. I actually feel better getting out of the regular routines at home, because that is always when I would smoke. I feel better, and hopefully won't gain as much wieght from quitting.