Wa alaykum assalam,
Sister, you are considering your religion and the purpose of your life at such a young age, ma'sha'Allah. I'd say that's more than what *most* people your age do.
I'm a couple of years older than you, and it was after a lot of soul-searching and whole lot more research that I feel fairly secure in my faith today. Faith, iman, can waver and fluctuate, we're not always going to have unshakeable faith and sometimes we'll question a part of Islam, such as the hijab or prayer.
I would say, read about Islam and specifically whatever you have doubts about. Islam never was a religion of blind faith and practice, Allah swt has provided us so much evidence and signs in the Qur'an, Sunnah and the world around us, it is up to us to search for them. Often we struggle with concepts such as the Day of Judgement or heaven/hell because they are part of the 'Unseen,' like the angels, the jinn etc, and the thing with us people is that we want to 'see' to 'believe'. There is a difference way of seeing things though.
Mankind has been placed on this earth as a test, sometimes people who do terrible things face little consequence, while good, pure people seem to suffer terrible injustice. Think of all those persecuted and killed in war-torn countries. If we were to just die and that was the end, then that would be so unfair, and directly contradict the idea the Allah swt is Merciful and Just. That's why belief in the afterlife is a pillar of imaan - without the afterlife there would be no point to even have a religion. On the Day of Judgement, we will be rewarded or punished for an atoms weights of good or evil. The court of Allah swt will provide the ultimate justice that could never have been achieved on earth. While physically we cannot *see* this, it's clear there *has* to be an afterlife, because as Muslims that's what we spend our whole life working towards, to be reunited with Allah swt.
May Allah swt strengthen your iman and keep you steadfast. Ameen. Make plenty of dua that Allah swt gives you understanding and just reflect on your place here on earth, it really helps.
Sorry for this essay of a post, lol. I hope it helps in'sha'Allah.
