You say you don’t know what to tell your mum, but my best suggestion is that you tell her the truth. A lot of problem gamblers keep this a secret from loved ones and that only makes things worse.
If your gambling is out of control then see if you can get your mum to look at the friends and families section of this forum for ideas on how to support you (and herself) through this. Depending on which country you are in there may be support groups (like GA / GAMANON) that you could attend to help. You will find lots of information on this site and you can also come to a support group (times shown in that section) to get more 1-2-1 advice. I wish you well.