"You'll end up really disappointed if you think people will do for you as you do for them. Not everyone has the same heart as you." PREACH IT to the grave. If you're somebody always giving people the benefit of the doubt or looking for the good in them, then you know all too well about this. But, just because people don't have the same heart as you doesn't mean you should stop being who you are.
You have a HUGE heart which means you truly care about taking other's feelings into account even if it means putting their happiness before yours, that's just WHO you are. I know it can get discouraging when people turn out to be something they aren't (especially after multiple times), but that's just life and not everybody holds the same values as you. Some people are selfish, valuing their selves and needs before others, whereas you could never even imagine putting yourself before a loved one possibly even a stranger (if you're SUPER kind), but to each his/her own.
No matter how much disappointment you face, keep doing what you're doing. These "people" and their selfish needs will eventually be weeded out of your life and get what's coming, but the people who do matter will notice the little things you do for them. They WILL remember how much you cared and those actions will not go unforgotten. I am not trying to mislead you because you will find people who have just as good a heart as yours and when you do, NEVER let them go. These individuals will reciprocate the actions you give them which is something to really cherish and be blessed about.
So keep having a good heart and showing it to everybody you can. Keep taking the time to make sure the ones you care about are happy, but don't forget about yourself too. Most importantly, keep looking for the good in everybody because sometimes all we need is someone to believe in us.