Hi Marly, that is a very interesting question. "How can I be really free of resentment in my heart."
It seams to me, human feelings can get the better of us, even when we know we should not dwell on resentful and bitter feelings. We know they are wrong and even harmful. Yet, some how, they simmer away in the back ground. When it comes to forgiving someone close who has really hurt our feelings, causing anger, resentment, hurt, betrayal, etc, it can be very difficult.
However, in Matthew 18:21, 22, (NIV), Peter came to Jesus and asked, "Lord, how many times shall I forgive my brother when he sins against me? Up to seven times?" Jesus answered, "I tell you, not seven times, but seventy-seven times." Here, Jesus is talking about a freely forgiving spirit, a willingness to forgive and not hold grudges. This begins by making a resolute decision to forgive them and managing that decision. Over time those feelings will keep surfacing, but as long as you keep reminding yourself of that resolute decision. Eventually those feelings will catch up with that decision and start to fade. Forgiveness begins with this resolute decision to forgive. You cannot hope to use your feeling to make such a decision, because of there fickle nature, they will keep changing.
Make a resolute decision in your heart that you have forgiven them, then ask for help from God's holy spirit to help you manage that decision. Over time, those hurting and betrayed feelings will fade and die. But, be prepared for your feelings to have a few fits before they die. Keep reminding yourself of the decision you made and why you made that decision, and eventually your feelings will get used to feeling differently regarding that person and what they did.