So let me understand this. You're in love with your cousin's sister-in-law, who lives away and you send letters to one another?! More to the point, you are 16, not 60, and I feel that you could get a boyfriend closer to you who you might actually be able to see frequently. What it sounds like is that he is your best-friend, which is fine. An ex-girlfriend of mine is my best-friend, but we split up as our closeness was causing us to bicker.
My advice to you is, yes, let him go! BUT still keep in contact through the post as best friends, this will then better the both of you. You cannot keep it going through fear of him self-harming; it's not good for you!