... but you might as well get it over with.
1. No matter how much you care about someone, no matter how well you treat them - they still might turn around and treat you like garbage. There is no point in beating yourself up about it if they do. People sometimes do shitty things. Sometimes they don't even realise how shitty the things they do are. It hurts like shitty death when someone you loved and trusted pees on you from a great height, but the reality is, you didn't make them do it.
2. Ignoring your own feelings will never lead you anywhere good. Pretending you don't have them is even worse. Sometimes you can't or shouldn't act on your feelings. But you should always acknowledge them. As much as you don't want to do something inappropriate, pretending it isn't there is not the way to deal with it. There is nothing I am surer of than this, pretending you don't want to do something when you really do is a sure fire way to end up doing it. And then you end up feeling like a guilty tool, and you probably deserve to. Even if it's just a piece of chocolate cake calling to you.
3. A friend will tell you what you want to hear, a good friend will tell you what you NEED to hear. Don't punish them for telling you what you didn't want to know. Sometimes you have to tell someone something awful. You have reason to suspect that they are about to do something collosally stupid. An epic mistake, you know they are going to regret. Telling them is hard, because you know they aren't going to want to hear it. But if you love them you do it anyway. And then they go bananas and won't speak to you... don't be a dick. The people who love you enough to tell you when you're about to derp it up chronically, they're the ones you want to keep around.
4. It is never, ever to late to say "I'm sorry". Even if you think the other party doesn't want to hear it. Time passes and we talk ourselves out of apologising. They're over it now, it would just be bringing up water under the bridge. If it was crappy enough that you should have apologised for it, you still should.
5. If you think your partner might be cheating, don't just accept it when they swear they're not. I know this sounds like bitter old woman speak but listen to where this goes. If you have a trust issue with your partner, if you feel like they might be cheating or you feel like maybe they're heading in that direction: that feeling doesn't come from nowhere. That feeling does not necessarily mean that you're right and they are a cheating scumbag but it does mean that there is a problem in your relationship. Don't just brush it off.
6. Sometimes someone in your life is more trouble than they are worth. But sometimes the reverse is also true. Some people are worth even quite a lot of trouble.
7. Doing nice things for people in the hopes that they will do nice things for you often leads to disappointment. I'm not suggesting not bothering to do nice things for people, just do it without expectation. You'll be repaid in feeling better about yourself. And it makes those occasions when you are repaid in kindness, that much sweeter.
8. Douchebaggery should not be answered with more douchebaggery. It may make you feel better in the short term, but in the long term you just end up with a big pile of used feminine hygiene products. Wow. Gross. The point is still valid, even if I slipped in a horrible and weak gag.
9. The standards you hold yourself to are not a reflection of the way other people see you. It's the other way around.
10. The world is full of differing opinions, yelling "YOU'RE WRONG" at someone, has never worked. If you cannot have a rational discussion about your differences you should probably just shut up.