I think one reason why breaking up and/or growing distant from certain people is so painful is because we think we have to stop loving those people.
We separate paths for many reasons in life. Sometimes we simply grow apart. Sometimes our life situations are such that remaining close would cause more trouble than it's worth. Sometimes people need to get their shit together and need time and space to do that (sometimes the rest of their life). Sometimes we acknowledge that we need to go on our own spiritual journey, get our own shit together, and change our own bad habits.
But whatever the reason we choose to emotionally and physically part from people we still love, it often hurts. Sometimes there's nothing that hurts more...
In my personal experience with this, I've come to realize that you can't stop loving people if you have ever really loved them. You can't "un-love" them the way you can "un-friend" people on Facebook. Oh, sure, you can stuff the truth down and deny it. You can try to tell yourself that you don't care, but that's when it retaliates the strongest. That's when it comes creeping up to tap you on the shoulder, for instance, when you hear a certain song or experience some other trigger of a memory of that person, and it hits you like a bullet to the heart. But you also know that things would not work out if you tried again with that person. So you're stuck in a Catch 22 of Mind vs. Heart.
The trick, I've discovered, to handling this kind of battle is to stop fighting it. Now, I'm not saying to give into grief and woe-is-me thoughts and feelings until you are completely depressed. What I'm advising is to let yourself keep loving that person, knowing that you made a choice for your good or for the other person's good, but let yourself keep loving. Doing this takes the sting away, makes it hurt much less, at least in my experience.
Resisting the love we still feel and will always feel is what causes the most pain. So don't resist it. Accept your situation and that you are apart for a reason (maybe a million reasons), but also accept that the love for that person will always be there.
Because it's cliche, but it's true: true love never dies.