Your girlfriend dumped you for being a brony? That's ridiculous. I was actually dating a guy almost a year ago, who I had to explain the whole brony thing to, and he still didn't get it, but he at least accepted it. It's not for everyone, unfortunately. I'd probably be psyched if I was dating someone and found out they were a closet brony or something, but I've found dating people with little to no mutual interests is hard, and the relationship goes stale fast. I'm sorry you had to deal with that, as I could maybe see friends being potentially put off, but if you're dating someone, you shouldn't dump them over something so frivolous. That would be like you dumping her because she liked oranges and you loved apples.
I wouldn't be completely dissuaded and "dump" the fandom, though. It's something you love, and other people don't have the right or privilege to make you sacrifice the things you love because they're unwilling to accept/understand it. Perhaps as a minor solution, you could just suppress your power level with your non-brony friends and make a few that are, or at least find meetups/cons in your area, or possibly look around the anime club, if you're at school and if you have one.
And if you have had any bad experiences within the fandom, they're easy to brush off. I've met so many wonderful people through MLP, and out of all of those, maybe two people have ever left bad tastes in my mouth, but I just move on from it. I'd rather brush some jerk off because I know I'll met at least ten awesome people in their stead.