Asalaamu alaykum and thank you for your question, sister,
I can appreciate your feelings and concerns on this topic. At your age, something like this could be very confusing.
But I want to assure you: merely learning about the religious beliefs of others is not, in any way, putting your own faith in danger.
Islam: The Pure Truth
Allah himself answers your first question on why there are so many different religions:
“Mankind was one single nation, and Allah sent Messengers with glad tidings and warnings; and with them He sent the Book in truth, to judge between people in matters wherein they differed; but the People of the Book, after the clear Signs came to them, did not differ among themselves, except through selfish contumacy. Allah by His Grace guided the believers to the Truth, concerning that wherein they differed. For Allah guided whom He wills to a path that is straight.” (Quran 2:213)
So, you see, initially, all mankind was just One Nation, One Religion, and Allah sent messengers to us.
Some of us chose to follow their messages, and others chose to deny.
Think of The Ultimate Truth (monotheism) as a giant lake, and each faith is a river which began as pure but was later tainted with untruths.
Islam is the only river among those which is totally free and void of any defects, but that doesn’t mean that other religions are totally void of truth!
The Truth is the Truth wherever you hear it. One can find many beautiful truths in Taoism, in Buddhism, and in the Abrahamic Faiths.
Indeed, if religions contained no trace of truth, human beings would not gravitate toward them! It is in our nature to seek out the Truth, after all.
Allah Gave Us Free Will
Moreover, Allah tells us clearly that it was well within His power to create us all totally submissive to Him, all following One Religion, and He chose to give us a choice:
…If Allah had so willed, He would have made you a single people, but (His plan is) to test you in what He has given you: so strive as in a race in all virtues. (Quran 5:48)
Allah could have created all people to be like the angels and only worship Him without question or confusion.
But Allah gave human beings a free will, unlike angels. And human beings are given the choice to choose the path they will follow.
We Should Learn About Other Religions!
As regards your intense emotional response when someone brings up the beliefs of others, this is an issue you must address.
We, as Muslims, should be very aware of the basic teachings of the religions of others, if not for any other reason than to have respect and understanding of other people.
O mankind, indeed We have created you from male and female and made you peoples and tribes that you may know one another… (Quran 49:13)
So, the fundamental reason there are so many different religions, races, tribes. creeds, ideas, and followings out there today is because Allah wants us, as human beings, to seek out and learn from those who are different than us.
Indeed, none of us grows and improves ourselves when we only surround ourselves with people just like us, who think just like us.
Let’s ponder this more deeply: why are so many people in the world so afraid of Islam? Because they don’t have any idea what Muslims believe!
How can we expect the world to learn that terrorism is not ok in Islam if we are not also willing to learn the basics of their religions?!
Protect Your Faith—By Learning Other Religions
The most dangerous thing for your faith, sister, is to be naive to the faiths of others.
Imagine that someone comes to you with what sounds like an absolutely perfect lifestyle, something that is very attractive to you.
As a former Christian missionary, I can tell you that one of the primary methods used to convert people away from Islam is to take advantage of their lack of knowledge of both Islam and Christianity.
Tell me: if you do not know at least the basics of the religion of those people, how can you stand firm in your own faith?
If a Christian tells you that Christianity is monotheistic, too, if you didn’t know about the historical roots of the lie of Trinitarianism, you may actually come to believe them and see Christianity as just as pure in truth as Islam.
Nothing in all the world could tempt me to leave Islam, primarily because I studied all the major religions fairly in-depth before embracing Islam.
I would encourage you, sister, to deepen your faith in Allah by learning more about Islam.
Prepare your heart for eventual tests by informing yourself of both the truths and falsehood contained in other religions.
Allah made the truth very clear and obvious from falsehood, so you need not worry that you’ll be led astray, so long as you cling to Allah.
I hope this answers your question.