First of all, I have to tell you that I am really admire you for writing in---doing so really does show a lot about how much you care for your mom...And trust me, you're not alone.
When I was little, my father also used drugs. It seemed like the more he became involved in it, the more my family fell apart. I cried myself to sleep many days because I hated seeing the effects drugs had on him. He was often irritable, irresponsible, and just, well...not himself. He would also tell me the same thing your mom tells you, "I'm not too far in it and I can ALWAYS quit." Yet, sometimes it felt like that "quitting day" would never come. After a few years of painfully watching my dad literally fall apart, I learned the ultimate secret...We shouldn't hide our feelings from our parents!!!
As kids, we often tend to think that there is absolutely nothing we can do to help our loved ones. However the truth is, there's A LOT we can do! First, I learned to "EXPRESS" myself. Don't "bottle up" emotions concerning your mom -- time let her know how much you hate her use of drugs. Secondly, be "PERSISTANT"--- I never stopped telling my dad how much his drug use was hurting me as his child, as well as tearing our family apart. Also, keep researching the facts about drugs. Let your mother know that meth is addictive, and users can develop a tolerance quickly, needing larger amounts to get high. In some cases, the user won't even eat or sleep. Meth can also cause strokes, heart attack and even death.
Really stress to your mom the effects meth can have not only on her life, but on yours as well. Tell her how much you love her and need her to FULLY be there for you. Parents tend to have "soft spots" for their children's concerns. However, we have a way of overtaking that. So, what you have to do is slowly kill those "barriers" with a mother's love. There's a lot hope that she'll come around...My dad did COMPLETELY! Those who meet him now would never think to imagine his past...Just keep hoping, praying, believing, talking and LOVING.
Finally, one additional thing. Sometimes our parents need more help than what we can give. So, if you ever feel in danger or feel your mom can not stop using meth, please tell another adult you trust. Even though we kids have influence, your mom may need more professional help.