I'm sorry you have experienced this distressing situation. At 15 years you may not yet have learned that the world is full of people who are frauds - even those people who say they love us and whom we love in return can often turn out to be double dealers or liars.
But another lesson you will learn as you grow older is that humans make mistakes. They make bad decisions, they make selfish decisions and the best we can do is forgive them and try to move on.
You say that you don't understand how your mum thinks she is right in any way. I am certain that your mum actually does not think she is right. She knows what she is doing is wrong on a lot of levels - not least that it is hurting you - but what she is doing makes her feel good in herself. It makes her feel desirable, loved, special, like a teenager again, and that is very very tempting for a woman (or a man) who sees the aging process beginning to show.
So although you know that what your mum is doing is wrong, try to hold back on your judgement of her because when things go wrong - as they will, they always do - your mum will need you there to help her get through the pain of coming down to earth with a bump.
Then it will be your turn to show maturity, responsibility and compassion. Will you be strong enough to step up to the plate, and support your mum when her fantasy world comes crashing down around her? I really hope so.