Wow, that was a really long story.
I'm sorry that you wound up getting hurt and taken for a ride. It's always a very difficult thing when you've opened yourself up to someone and they take advantage of you in return. "L" is an example that you'll remember for a long time, and it will probably influence the friends you make in the future, i.e. it'll take you more time to trust them.
In terms of closure, though, I think you have a very good handle on the situation. You said, "I think “L” has some serious issues. She lives in a world of lies and deceptfulness. It seems to make her feel better about herself but it makes everyone around her miserable." You nailed it on the head. There are people like this out there, and they suck, and all that you can really do is move on. I don't think there's any point in reconciling with her. She clearly doesn't really have much concern for anybody but herself.
The one thing you should take with you, especially because you're starting college in a few weeks and will be meeting new people, is that not everybody you meet will be like "L." It is normal keep your guard up for a while, but don't limit yourself when it comes to meeting people and making friends. You seem like a really likeable person and if you work up a little bit of courage to come out and talk to people when you get to college, I'm sure you will make tons of friends there. The more interaction you have with people, most likely the more you will be able to notice the characteristics that a person like "L" exhibits and then avoid them in certain people. Life is a series of extreme highs and lows, and unfortunately, the "L" thing was an extreme low. But don't limit yourself from any interactions at all for fear of the low moments. That's what life is all about - experiencing both and learning from all of it.
The other thing I would suggest in the future, although it will be tough, is to not play the middleman games - i.e. all messages between you and "M" being passed through "L." Although you are shy and it is always difficult to find out what someone like "M" is really thinking at first, the middleman thing not only opens you up to problems (like the ones you wound up having), but it ultimately annoys all parties involved. That's something you should definitely leave behind before you go to college. A little bit of it is okay at the beginning, but ultimately, you will be better off if you take your balls in your hand and go for it. It may not always work out the way you want it to, but I think the rewards in the end will be worth it.
Good luck, hang in there, and I hope to read posts about your college experiences in the future.