I'm struggling with trust issues, not so much in romantic relationships but with friends.

The back story: almost two years ago, I ran into my neighbor across the hall (he had just moved in) and we hit it off pretty well. He was struggling so I helped him out a little bit, but we were good friends and just hang out or maybe go to dinner. Well he kept asking for money, a little bit here & there, and paid it off. After he was evicted, I let him stay for a time at my place, and I asked him to look after it while I was away on vacation. When I returned, I found out that a $400 disk drive for my computer that I received for a birthday present was gone. After I decided to go back to school, he kept asking me for money and favors at all hours, including going into the scary parts of town at late hours (extremely dangerous situation). Twice I cut him off but he threatened suicide and got hot at me saying I was the only person who could help. He always needed money for his 3 kids who lived with his crackhead ex-wife (and yes she was that, her place reeked of drugs) but I wonder where the money I loaned him went to. He literally broke me financially and emotionally, to the point I considered suicide. When I told my family that I was going to take my life, they came to take me out of town for an intervention of sorts, and I left my keys at home. Well the scumbag took my car without a driver's license (I had let him borrow it but I didn't know he didn't have a license), and he got busted and sang to the cops that I let him use my car, so I had to cancel a vacation and spend family money on lawyer's fees to get the charges tossed.

Sorry for the rant, but the point is, I'm afraid to let anyone into my life because I worry about their motivations. This was someone I had trusted but took full advantage of me due to my belief in helping out others. I just have a distrust of people when they want something from me and I'm really creating a shell around myself. Is there a way to see the good in people?