I worry about this, too. I'm the only child of a single mother. She's devoted so much of her life to me. My suicide will devastate her.
The only consolation, I think, is that she is very aware of my struggles. While she will be terribly sad, she will not be shocked. She may even, with time, come to understand it as an act of mercy. Of course, I can't know that for sure, but it's a possibility.
Have you shared how you're feeling with her?