Why do I do this?

Why is it that I offer to help "friends" in need and end up ALWAYS regretting it and getting screwed?!?!? I have a family of 5, why do I feel the need to let "just one more" stay at my house? The last time I did this, I honestly didn't regret it. "Always" is an exaggeration. He didn't do anything out of line. He didn't abuse the situation. He helped out around the house and helped me rein in the kids when needed. HE was a real friend, and VERY grateful. However, this person..... ooooh this person.... I do her laundry, cook her meals, wash her dishes, sweep her hair off of my bathroom floor, listen to her sob stories, the list is getting longer and longer. Oh! Let's not forget!! SHE IS DATING ONE OF MY BEST FRIENDS (who still asks about you MBG) AND NOW THINKS SHE JUST WANTS TO BE ALONE FOR A WHILE!!!! The last time.... wait.... last TWO TIMES... she dated one of my friends, it ended up in freaking Armageddon and I lost a 20+ year best friend out of the deal and Jeremy lost a good friend of his (of which he has few). She takes my clothes without asking. I don't have a ton of clothes, ya know. She has a TON of work clothes down there and she feels the need to use the two nice outfits I have in my possession?!?!?! Oh and let's not forget! I had set out my things so that I could get up and get ready before the kids got up, trying to get into the school routine, and she came home from work that night and said (in a snide tone),"Thanks for leaving my deodorant and perfume out this morning so I didn't have to go in your room and get them!" Translation: I left MY deodorant and perfume laying on top of my clothes and she felt the need to use MINE AGAIN because she 'is too lazy to go alllll the way downstairs and get hers'!!!

Ok. I have to stop now or I never will. SUGGESTIONS?!?!? Because I can tell you that I am about to blow up and we ALL know what happens when I reach the breaking point!!!