I think there's a huge difference between a sudden break off by friend who does something dreadful (e.g. bad mouths you, goes behind your back, etc) and just a general drifting of contact though.
With the former, you obviously have justifiable complaint and the fault is all with that ex-friend (and you also have the added bonus of relief that they're out of your life!)
With the latter, I feel that you can get too worked up if you dwell on it - often you read far more into it than is actually there. It could just be she's feeling antisocial at that point in time, or life circumstances have changed, or you both just don't have that much in common anymore. They might not even be aware that they haven't had much contact recently. Or you may have completely misread cues and they were never really that in to you.
By dwelling on it, you can get het up and see slights that really were never there. Worse, you can end up poisoning future relationships by coming across as too "needy" or setting up your own self-fulfilling prophecy ("woe is me, I never seem to hold on to friendships for long these days").
Far better to think "yeah, that friendship was great for that moment in time" and move on. Meet new people. Don't expect one person to be your be all and end all, it's too much pressure on them and we all have different lives and different commitments which can vary over time.
I do sometimes wonder if we cling on to an outdated ideal of friendship harking back to primary school "you're my bestest friend ever and we have to do EVERYTHING together" then fall for the same old misery a short time later when said "bestest ever friend" has moved on to another kid in the playground.