Gender
Gender is usually just a euphemism for sex (male or female). It's also used for words in other languages.
But recently there is gender ideology, which talks about gender as if it was a thing on its own that varied independently from sex and is more important than sex. Gender ideology is a confusing and life-ruining ideology that I want to spare you from. Your sex is your sex, you are not how you feel. Your best chance is to deal with the reality that you're of a sex and not of another. I think most (or all?) people are fine if they do this.
A lot of girls want to block puberty and try to be boys. They do this for several reasons like..
- They're depressed or have an eating disorder and are told that becoming a boy will solve their problems
- They are scared of hitting puberty, starting menstruation, and getting unwanted attention from boys and men (unfortunately that last one is a real danger)
- They want social acceptance or acclaim, or to be popular or even famous
A lot of boys want to block puberty and try to be girls. I think a lot of this is because they find the idea of being a girl sexy. I think a lot of it is just to be interesting or to get social approval or acceptance. Less scrupulous reasons may include wanting to cheat at girls' sports, to see girls naked or expose themselves in change rooms or locker rooms, or to go into women's bathrooms.
Professionals such as therapists and doctors often recommend transition because they think they are a good person, or because they get bucketloads of money from it, or sometimes because it's almost illegal not to. Parents or schools may encourage transition of kids because, they believe in gender ideology, or just selfishly want to look good or interesting to others.
Puberty is hard. I know, I went through it too. But it's worth it. You get a better body and a better brain, and you get to be an adult sooner or later.
In any case, becoming another sex is impossible. It cannot happen, you can only pretend. If you are a boy you will always be a boy or a man, and if you are a girl you will always be a girl or a woman. And there is nothing wrong with that.