Any relationships (family, friends, or romance) takes time to build. Sometimes, even when you feel the relationship is genuine, things happen and your opinion may change. You don't always have to see people in order to continue the friendship with them. My absolute best friend has lived states away for four years now; we see each other once a year. With modern technology, there's so many ways to stay in touch. I have many other friends that live hours away as well. With everyone's lives being so busy, we literally have to set call/chat dates. This is something you could try with your friends as well. It gives you something to look forward to and it will help you stay in contact.
As far as your boyfriend... I don't think he can understand right now. Please be aware, that is truly my opinion. As you know, I'm on the "other side" of things. My ex didn't try to explain to me, she just cut it off. Did it hurt? Of course. Just like love can make you blind, I think that pain/hurt can really distort reality and perceptions. She could sit me down and explain till she was blue in the face, but just hearing that we couldn't be together right now was overwhelming to the point that I don't think I could really listen or begin to understand. Like Jamiee said, writing a letter may be your best bet, especially since you like to write. Remember, one day you may be able to explain. The most important thing right now is that you know you can't explain everything to him, but that it must be done for your independence. This may be something to bring up in counseling.
Good luck, keep posting.