Can you tell me how to tell people i'm vegan without them thinking i'm self-righteous?

Be honest: Whenever you do something that gives you the moral high ground, you'll be called "self-righteous." Whatever your motives, here's your best approach. "It's just my personal preference. Please tolerate me." And don't argue. I also trust that you're aware of the real science -- it gives you a moral, but not a demonstrable health, benefit. Keep it up as long as you feel it's right for you.