My 2 cents worth, here Mike is:
If your mom wants to smoke, it's her business and choice. You love her whether she smokes or not right? and that is what is important that you show her how much you love her, whether she smokes or not. You may just have to remove yourself from her space while she smokes her cigaretts, and politely excuse yourself so as not to be offensive toward her. Remember, she is your mother and you only have one mother. Let her live her life to the fullest while she can because from what you say, she is not about to change, and nagging her will only make matters worse between you.
You don't have to feel guilty about anything and all you really can control in this situation is what you eat and what you do with your life. Set your priorities which may include cooking separate meals for the two of you, and take time out for yourself for your exercise regime.
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Thank you for your advice. And yes, nagging her doesn't solve any problems. I've tried....it doesn't work! haha!