Hi Homewardbound,
You seem like such a compassionate person - many kudos to you. I know that as Christians, we can actually expect that there will be times we'll be taken advantage of, but it always hurts when it happens and it makes me sad to read about this happening to you.
I was torn for many years over how much we should help other people. After all, the Bible tells us, as you pointed out, to go that extra mile, turn the other cheek, and "Give to the one who asks you, and do not turn away from the one who wants to borrow from you" (Matthew 5:42.)
This is just my own experience, and yours might be different, but I've found that God will give me direction regarding various situations. I once had a "friend" who was asking me for money, and this person even hit me with the Bible passage, "Keep asking, keep seeking, keep knocking" (Matt. 7:7-12), and told me, "I'm supposed to keep asking, so I'm asking you." The person also implied that I was wrong for not meeting their request, because they were obeying what God had commanded them in the Bible.
I asked God about this, and felt that the answer He gave me was, "Yes, (this person) is supposed to ask, but they're supposed to be asking ME, not you." I relayed this answer back to the person, took all the expected hellfire and damnation reaction one would expect (being told I wasn't a real Christian and how could I let Satan enter my heart, etc.), and promptly walked away from that situation and that person.
The Bible also tells us, "Do not be fooled. Bad character corrupts good morals" (1 Corin. 15:33), and while I'm certainly not trying to claim that I am any better than another person, there are others I want to strive to be more like and others I definitely do not want to be like (such as those who use the Bible and faith as manipulation tactics.)
I can relate very well to some of what you're saying because as a chronically single person, I run into this all the time. I can't tell you how many times another single person or someone going through a hard time has latched on to me because they figure, "You're single, you don't have kids - you should have all the time in the world to listen to ME, help ME, serve ME, and keep ME company." And then, once their situation gets better or they find someone, they disappear, except when they need something again or need someone around them to try to make them look better.
It's hard not to be bitter when people are using you like a Kleenex. They cry their eyes out, tell you all their problems and use you for comfort without ever asking or caring about you as a person, then throw you away once they feel they've gotten everything they can squeeze out of you and move on. Or, they'll come running back as soon as things go bad with their latest significant other (even though you've warned them of red flags the entire time.)
Something that really helped me was the story of the 5 wise young women and the 5 foolish young women who were preparing for a wedding party (Matthew 25.) The 5 wise young women had planned ahead and prepared, bringing extra oil; the 5 foolish young women who didn't prepare thought they could just ask the wise women for some of their oil. But the foolish women were not only told no, but they were also shut out of the wedding party.
I know the aforementioned parable is about preparing for the kingdom in heaven, but I really believe that God was leading me to understand that this is part of our preparation here on earth as well. There is a certain point where even God says, "Enough. You've had time and you've had opportunities and you've wasted them all, so now you're going to have to learn on your own."
This has helped me to make a distinction between helping or enabling people, and I have tried my best to cut down or cut out the enabling, which has made serving the people who aren't trying to just use someone a whole lot easier.
I certainly understand that there are times when we are to continue doing good to someone who hurts or wrongs us - it's just that I used to think I had to be nice to all of them, and I believe God actually confronted me to stop enabling various cycles of behavior.
I'm sorry for this long post, but I hope this might have been of some help.
God bless you, Homewardbound!
I pray that God will comfort you and send you people who are serious about being true friends in your life.