I think it starts by valuing yourself. If you could do anything, what would you want to do with your life? Just because you are a SAHM doesn't mean you can't have other dreams and goals that you can be working towards now, even if it's just in a very small way.
Of course I don't know your whole situation, so you can take this or leave it, but I'm thinking if you make a conscious choice to make time for things in your life that are separate from your wife/mother role, it will help--you could start a hobby, something artistic, dancing, join a gym, go to concerts...or educational/professional goals, like taking a class or even starting a weekend job to earn some money, if it's something you want and there's a way to make it work.
Journaling can also help--taking the time to make lists of what you're grateful for, and lists of reasons why you love yourself and what your strengths are. Just change your thoughts and that alone will make you feel better.