How do I heal from being alienated from my children?

I feel for your loss, but you cannot have been entirely blameless. You went into the relationship with your ex with your eyes open and when things began to go south on you, there must have been some indication that he wasn't going to let you win in the end. Unfortunately, you didn't prepare for war before you alienated him and that was your downfall. Your children will seek you out in the end; you just have to be in a place where you can receive them warmly and have a succinct explanation and apology for your absence. Then, move on and try and make amends with your new family: your grandchildren!