OP - I'm sorry you're having such a rough time. Relationships do take a lot of work, but it can feel one-sided when the other isn't pulling their weight.
I'm not a guru on relationships, but this is just my two cents on somethings:
Don't shower people with gifts/money/etc. with the intention of forming or maintaining a relationship. You will feel used and just make yourself frustrated over the situation. In that same aspect, don't give and automatically expect something in return unless it's a loan. I'm not saying gifts aren't a lovely thing, but too much of a good thing has its consequences.
If you go out to dinner or movie and pay for it - try not to get too spiteful if it doesn't work out. It's basically your investment into a relationship that went sour, if you want to think of it that way. Cut your losses and start over.
Giving someone a ridiculous amount of attention with compliments, having long insightful conversations, allowing that person to confide in you -- all of which are very nice things to do, won't guarantee that person will reciprocate your feelings. Be as directly honest as you can with yourself and them without being cruel. Don't scheme and manipulate others to get the results you want.
Be wary of self-proclaimed nice people -- hell, just be wary of people and give yourself time to really get to know them. Nice people are amazing, but don't think for a minute I don't know that people are humans, and all humans have a nasty streak (no matter how small); some are just more evident and obvious about it than others.
Most importantly, I think we teach people how to treat us. That's really one of my biggest problems, there are particular people that I let hurt me all of the time. Things won't change unless I take up some kind of action -- that may or may not apply to you as well.
