Sometimes I make a mistake in choosing that person. I knew deep down that one person I let get close had some issues - but nothing had been done to me so I chose to give this person the benefit of the doubt. That was a mistake.
The same quality I have about loyalty goes the opposite direction as well. Once spurned, I don't forgive. It's just part of my nature. I'm not terribly proud of it, but I feel everyone gets one chance to piss me off, and depending on the gravity of the situation, we either remain friends or not.
Mind you, I have plenty of friends who I've fought with and remained close to. Being my friend doesn't mean agreeing with me. It means respecting me and treating me with respect. It's what I give in return.
So when someone comes along after many years of friendship and disrespects me, then I can't be friends with them anymore. It means I can't talk to them anymore. It means I can't trust them anymore.
Inhale. Exhale. Put one foot in front of the other and move forward. The sun will still shine. Night will follow day. It's all good.