Haha yea, crazy right? Most people go through life never realizing that, saying "I'm unhappy because this or that happened," always believing that their own happiness is outside of their control. For them, happiness is at the mercy of a roll of dice--either life is good to you or it's not. But that's completely not true.
You're right though. You will still feel things. If a girl dumps you it will hurt, but once you've healed enough you can decide whether you will stay mad at her, hate yourself, tell yourself you're worthless, or... you can choose to forgive her, choose to recognize that even if one girl didn't like you, you're still worth it and someone else will, or choose to become a better person for next time. One of those roads leads to endless bitterness and misery, the other keeps you moving forward and gives you the will to live for tomorrow.
The truth is, those who believe they deserve love are the ones most likely to find it. I don't mean just romantic love, but wholesome friendships too. This is because they act on the idea that love is possible, so they pursue it more openly and with more positivity. Those who never think they deserve it will never open themselves to it. So you always have a choice. Your choices may fall flat and life may punch you again, but you choose to believe in yourself to get back up again.
When I was a teen I had similar questions that you did. I was pretty depressed and had trouble finding ways to be happy. Someone recommended a book called 7 Habits of highly effective teens. It talks a lot about the internal vs external control (and I'm still quoting that book years later). I realized that I was viewing life entirely backwards. I highly recommend it! I'll even buy it for you if you want.
Oh one last thought. Sometimes we know we have to forgive, or we know we have to choose to be positive, but our emotions are so strong and complicated that it's hard to know how to unravel the mess and move forward. Therapists are there to help us understand what we are feeling so we can. One of the best therapists I ever saw just let me talk and asked me the right questions to help me understand myself. It was pretty cool. Anyway here's the book link!
https://www.amazon.com/Habits-Highly-Effective-Teens/dp/1476764662/
There is nothing unrelaxed about what I am saying. I am simply saying it firmly so you stop ignoring it while continuing to ask the same questions, which is a repeating pattern with you.
Yes, that book and 7 habits of highly successful teens. Both lay the ground work of very important social paradigms that help you understand your own behaviors and how to identify toxic behaviors in other people. Even if you're not a teen, it's social gold to help you lay the foundation of a healthy paradigm. They cost very little.
https://www.amazon.com/Habits-Highly-Effective-Teens/dp/1476764662/
https://www.amazon.com/How-Win-Friends-Influence-People/dp/0671027034/
By the way I got these principles from here
https://www.amazon.co.uk/Habits-Highly-Effective-Teens/dp/1476764662
first, and then
https://www.amazon.co.uk/Habits-Highly-Effective-People/dp/0684858398