I think it's probably really hard to see that how things are for you right now -- not just at your age, and at this time, but also with the community you live in, including your immediate family -- is not how they always have to be.
For sure, outside of major cities, you're going to have a harder time finding what you're looking for: you know as well as anyone that people like yourself are usually rarities. As well, most high school sexual or romantic relationships just don't even go on long enough to usually have them be poly, since that tends to take time and a whole bunch of setup and education that's a lot more complex than the standard vague high school monogamy agreement ("we only have sex with each other" literally takes as long as it did for me to type it to create). So, even separate from other factors, that one alone tends to make poly relationships in high school pretty unusual for people.
What I'd suggest to get you over the hump, as it were, with this, are a few things:
• If you haven't already, get started on your life plan to live somewhere that's much more diverse and queer/poly friendly than where you are now. Same goes with a plan so you don't always have to live in a home with someone so unaccepting of you. What's that plan? Is it about college, is it about an apprenticeship, or is it just about trying to find a way to save up so that when you graduate, you can move somewhere else and get a job, etc? Whatever it is, take time to really hone it and stick to it. Just having a plan and knowing you are doing what you can to advance it can help you emotionally get through the time when you aren't there yet. You're so close to some autonomy and a big change: 18 is RIGHT around the corner.
• As I suggested a long time ago, I'd still encourage you to see what you can do to get to the closest LGBTQ youth group or center to you. I believe that's still this one:
http://lgbtcleveland.org/index.html I know transit is an issue, but if you didn't before -- or you did, but it's been a while -- why not call them and fill the in? They may be able to help you out, like by connecting you with someone else who attends their youth groups who could give you a ride. Every LGBTQ center there is is always going to have at least one person on staff who knows exactly how it is to be young and challenged in getting to community, because it's just how it so often goes for so many, so they are usually going to be pretty dedicated to helping get you to community if they can.
• You might also remind yourself that some of the experiences you are having sound a lot like most people's high school dating experiences. For instance, people having sex outside high school relationships is super common (probably because most people agree to be exclusive because they want to in the moment without an awareness that their interests will probably change way more quickly than they think). There's also often a good deal of people agreeing to romantic relationships when all they really want is something sexually casual. And some people just come in and then blow away. Being 17 and having had just one dating relationship that really clicked is, I'd say, pretty average.
I don't say any of that to dismiss your experiences, but instead to remind you that dating -- especially in the teens and twenties when everyone is so new to it, so new to their own desires and abilities, and everything is changing so often and so much -- is a crapshoot for most people like it has been for you, so this isn't about you sucking, nor about you having things about you that guarantee this is and will always be harder for you, or at least, as hard as it has been. In other words, there is hope, and some of that comes from some pretty simple stuff, like people just gaining more maturity and experience and getting to know themselves better.