Hi Zen,
I'm sorry you're feeling sad. I will miss my Mom(died 12yrs ago) too. You obviously love your mom alot - so instead of feeling sad, think about all of the wonderfull qualities your mom had. In other words make mother's day a celebration of your mom - you could make a scrapbook, /go put flowers at the gravesite and even go out for lunch with someone special. Instead of feeling it's a sad day, think of it as a happy day, that you honour her memory. It's sounds like you both had a special bond and I don't think she would want you to feel sad. The more at peace you are , the more free her spirit will be free. Fill the gap that you have, with cherished memories and wake up with a smile on mother's day.