I'm in love with someone, however he's polyamorous and I'm very monogamous. To the point where it would hurt knowing he's dating anyone other than me - not only because of how I am but because someone I dated before cheated on me many times because I'm asexual. My brother keeps telling me I can't force myself to date him because it'll hurt me, but we're both unhappy now anyways so... Should I just suck it up and date him so we're both at least mostly happy?
No, you should not start a relationship that requires you to “suck it up.” You already know that dating someone polyamorous would “hurt.” Your brother is right. I know it sucks to not be able to date someone that you really want to date, but sometimes just being “in love” isn’t enough to make a relationship healthy and sustainable.
Take some space from this person, and start actively seeking out relationships that are going to meet your needs better. Spend time online and in-person in spaces that are tailored to asexual, monogamous people. Set up a dating profile that specifically mentions what you are and are not looking for. You’ll find someone who makes you happy to be with, not just unhappy to not be with, and who doesn’t require relationship terms that will cause you pain.