I had a mother just like that when I was a teenager. I brought home a red eared slider when I was 13. I had him in her house for 8 years and she never stopped threatening to throw it in a pond.
Nagging your mother, crying and begging is NOT the way to fix things. Show with your actions that you know how to care for an animal. Keep the tank in a corner of your bedroom that isn't immediately visible from your bedroom door so she isn't always looking at it and getting an angry reminder of it. Care for the animal to the best of your ability and use your own money (allowance, odd jobs, and employment depending on minor labor age limits in your area) to feed it. Keep it in good health. Know that one day you may need to spend your savings on vet bills instead of a new smart phone. Be a positive example of a snake keeper to your mother.