Does Christ's sacrifice "once for all" mean that we are already forgiven for every sin we will ever commit in the future, thereby eliminating the need to ever ask for forgiveness again?
Yes (future sins) but no, for we do need to ask for forgiveness. To illustrate, if you are married, of course you expect to be married for good. Your wife knows you well, and knew she was not marrying a perfect man. But certainly you will ruin your relationship if you don't discuss the things that hurt each other.
In the case of faith, this means that we can ruin our relationship with God, which leads to our faith weakening, which in turn can easily lead to losing our obedient spirit. And without obedience and holiness, no one will see the Lord (Hebrews 12)!
So keep working on your relationship with God in prayer, but do continue asking forgiveness from the one you have grieved. This is the only way to maintain humility (on your part) and closeness in the relationship.