Sometimes we have very strong feelings for someone who doesn't feel the same way about us. Teenagers are particularly prone to this, I think. sadly, unrequited love is something we just have to deal with. I don't know why people fall out of love, but they do. And, again, you hear about this sort of thing all the time with teenagers.
It is better to love someone who loves you back ~ on a long-term basis ~ and I am sure that you will find a Mr Right for whom you are also Ms Right.
Of course you will feel upset ~ that is normal ~ but so is falling in and out of love when you are young. Just think of all the songs that have been written on this very topic. You are not the only one going through this ~ and the sadness will not last for ever, so look forward to a future with the right person for you. You have plenty of time to find him.
Good luck!
