In short, a model custodial parent always: encourages a good relationship between the children and the other parent; never bad mouths the other parent; includes the other parent in all major decisions concerning the children whether required or not; keeps the other parent informed on all issues concerning the child(ren).
What is an unfit custodial parent?
In short, an unfit custodial parent usually: interferes with the other parents parenting time/visitation; bad mouths the other parent to the child(ren) or in hearing range of them or implies it; uses the children to hurt the other parent in manipulative ways; never informs the other parent of anything and keeps the other parent out of the loop of all decisions concerning the child(ren) until after the fact; tries to micro manage the other parents time with the children.
Life as it is:
A fit custodial parent puts aside all issues between that parent and the other parent (emotions, legal, money, etc.) for the sake of the child(ren) in the context of the child(ren). An unfit parent seeks to turn the child(ren) against the other parent. Children usually side with the primary custodial parent because they have to go back home to that parent and do not want to risk problems with that parent and hope the other parent truly loves them unconditionally and will forgive them.
In time, as these children grow older, and become adults, they usually grow strongest bonds with the parent who never sad a bad word about the other and never sought to manipulate them or use in their war with the other parent. The better behaved parent spends many more holidays with the adult child(ren) than the unfit custodial parent (as described above). So rise above for the sake of YOUR child(ren), and it will come back to you in the long run.