Have you ever had a friend that you've known for, let's say, 16yrs, and all of a sudden they started being a total ass?

I'm sorry for you! I've been through a bit of the same thing. I'd be a cliché if I'd told you you could do better and stuff(not saying you couldn't, but let's stick to what I want to say). You're right it sucks but you should talk to this person. Let him/her know how you feel about it but first keep your emotions together. When you start talking let them know you still like them(even though you might not even do so. but trust me, it's better). Then when he/she still hasn't given you the reason why, confront him/her with everything that's on your mind, and then let your emotions go free. It worked for me, so I hope this helps.

And when this person doesn't stop becoming heartless and insensitive, don't think too much about it. If you know the how and why about this person's changes, let it be. How bad this might suck, there's nothing you can do about besides talking it out. Sorry!

I hope I could help!