I am lucky enough to have never had to do that. But, even if I did, I wouldn't equate that with "unforgiven", because the point of doing so isn't to hurt the other person or prevent them from having a good life (you don't have the power to do that last thing, anyway), but to (smartly) protect yourself from being harmed by them.
Honestly? This is gonna sound harsh, but if someone does cut you from their life in an attempt to hurt you, or rubs it in your face or tries to make the process as painful as possible...that probably means that it's time to move on from that person.
I'm not saying they're worth less than anybody else, or even that they're wrong to do it, just that if somebody is using their affections (in any sense of the word, not just romantic) as a bargaining chip or weapon, then that's their prerogative and it's better if you, as the object of this, don't internalize that.
Respect their decision. Acknowledge what you did to make this happen, and sincerely work as hard as you can to improve yourself. Feel the guilt, and use it to motivate yourself to be better, not cripple yourself. Miss the person, feel that pain as much as you need to, and acknowledge that it's a natural consequence of what's happening and can be overcome with time just like any other negative emotion. But unless they're the doctor operating your life support system, one specific person not being part of the circle of people who care about you is never the end of the world.
(Sorry if that got preachy, but it's something I feel very strongly about and I think isn't talked about enough. If my little rant is seen by even one person who needs it, it'll be worth it. ^_^)