Aww, Kits. I'll start by saying I don't really have much advice for you seeing as though I am extremely new to poly, but I just wanted to say that I can really identify and relate to your story.
I've always known that I was interested in both guys and girls, but I always felt like I should put those feelings towards girls away. I also always knew that I had a ton of love to give, and thought I could be in love with more than one person. It took me a while to understand it, and for a long time I felt like something was wrong with me. Like why did I have this wandering eye even though I was happy with whomever I was with?
I did have a brief and unsuccessful poly relationship where I was in love with four different people, and it opened my eyes to all the love and jealousy and happiness and insecurity that can come with this.
Now in my second poly relationship, I am dealing with more of this jealousy (my partners, not mine) but at the same time immense amount of love and happiness.
By no means can I say I am an expert, but I feel like this may just take time. The best and truly minimal advice I can give is be true to yourself, no matter what. Don't press your desires away.
Part of being young and 19 is trial and error in relationships. And a lot of broken hearts. So be ready for those. From these two experiences you have described, I'm sure you have felt the disappointment, but just keep your head up and keep looking for your happiness. It will come!
Okay, now I feel like I'm rambling and have probably talked too much about myself, but I could just relate to your story and wanted to help in the small way I could!
P.s. I love cheesecake too! I mean obviously...