If I want to be a totally new person mentally what should I do to achieve that?

start to appreciate what you have instead of what you don't have. it helps a lot. it can be something little, like a gift you got from a friend to nature like the blue sky or the smell of rain on plants. even if you have the worst day ever, if you just remember all of the little things you appreciate, you can feel better in due time. i don't know your situation specifically, but it helped me

Do something you havent got to do in a while that makes you happy! Get in touch with some good friends and go out and do something, just treat yourself! You only live life once just go out of your way for a while and take some you time. Try and achieve some goals you've wanted to achieve trust me if you just go out of your way to be happier for a while you'll come out of it as a fresh new version of yourself!

You don't need to become a whole new person mentally to improve but think that you need to improve yourself without changing it. Sooner or later you will realize that you changed