It's hard, hard hard. Once you get used to not seeing them often, the worse part for me was the realization that our friendship hadn't meant to them what it meant to me.
These are people that I would have done anything for. We've been in each weddings, supported them through pregnancy/labor/childbirth, losing family members, etc. We were closely bonded. And now? It's like they never even knew me. No, hey how are you? How are the kids? I miss you, let's do lunch. NOTHING.
It still hurts to realize that what we had meant nothing to them, but as the others have stated when you do move on and form other friendships, you start to appreciate what true friendship is. It's awesome to have someone love you and support you even IF you have differing religious/spiritual beliefs.