I know how hard it is to tell your mom this secret. It's such a big thing to tell her. You have to basically pour out this incredibly intimate thing, and she may be dissapointed, or even lose trust in you. It's a difficult decision to make, but it shows bravery and honesty that you want to do it. I'd say the best way to go about it is to be straightforward. Tell her, "I kinda have something to tell you. I really regret it, but you're my mom so I kind of figured you'd want to know." Then just tell her. Go through with it, be completely open.
Now, your mom may react a number of different ways. She could be angry, which is a huge possibilty. She could be dissapointed in you. She could even ground you. Accept all forms of punishment. Tell her that she's right, that it was a huge mistake on your part. Don't defend yourself, don't become angry. I've learned from experience that the best way to deal with angry mothers is to simply let them get mad, and then let them calm down. It'll hurt for her to be so angry, but just deal with it for hte time being. Let her calm herself down.
Another reaction that she may have is sadness. She'll think that she made some mistake in her parenting. Assure her that this is not the case, that it was a lapse in your own judgement, that it won't happen again. Tell her that she's an incredible mother, and that you love her and she's done a great job in raising you.
She might reply in indifference. She might think that it was your choice, and although it might be a crappy one, it was your choice to make. Make sure you let her understand that you know you did something stupid, and that you take full responsability for your actions. Remember that she cares, she just isn't wasting time on a past mistake that you know you made.
I hope this helped.
Good luck, and message me if you need anything at all.
All the best,
Nora