I've seen questions like this, but I would like answers for my specific situation.

When I was fifteen, I lost my virginity to a rather older boy. He was eighteen. By law, it was wrong, and I know that very well. He is long gone. At the time, I thought he might have really loved me, but, I was probably wrong, and I will admit that. Telling my mom might have been the biggest mistake I've made about that. Long story short, she treats me like a whole different person. Now that I am with a male of a decent age, one year younger than myself, she thinks I am doing only sexual acts with him. She's threatened me not to have sex with him or anything of the sort. Her words remind me of my regrets and I do not wish to relive such mistakes. I feel like a bad person because I am no longer a virgin.

What do I do about this situation, how do I help myself to feel that I am not a bad person?

The 18 year old was the one breaking the law, not you.
A 15 year old is not considered mature enough to make decisions to have sex.
Many an older guy seems like such an idol, and full of promises, and you fall for it.
First forgive yourself. You are beating yourself up terribly here.
Once you can do that, you can only hope that you can slowly say something of the above to your mother, looking her in the eye, and saying it's a naïve young past and it's over.
Tell her not to blame herself for her parenting. Some parents can't admit to themselves that the punishment that they inflict on their children is just a way to try to deny some amount of guilt - but don't say that. Let her figure it out herself.

Joypulv is right. Parents get angry over things, but they get over it. Don't forget although you were both in the wrong, the 18 year old should have been more responsible. Forgive yourself. Stop giving yourself 40 lashes over something that's done and dusted. It's better at 15 than 12, like some people in the media. Just let go of the past. Forgiving yourself and trying to personally get over it is the first stepx
Good luck and all the best xx