How Can I Know If I’ve Truly Forgiven Or I’m Holding Resentment?

Forgiveness is an act of agape, the highest NT Greek word for love – sacrificial love. This means genuinely wanting the best for the offender. Without agape, forgiveness is impossible.
Forgiveness will primarily therefore be praying and acting for God’s blessing, healing, restoration, transformation to be experienced by the offender.
Forgiveness can be genuine in the absence of philia: the Greek word for a feeling of affection towards the offender. We can decide to show love even if we don’t feel affection.
Forgiveness must however be sincere (“from the heart” Matt 18:35) in wanting the best for the offender.
Forgiveness will not necessarily mean the hurt or emotional impact of the offence is removed but it does mean deciding not to act out of that hurt, which would be resentment. i.e. resentment is withholding agape from another person because of the hurt felt.
Forgiveness is not incompatible with expressing the emotional hurt to a third party for constructive reasons: to help with the healing or the process of showing agape to the offender or because the third party genuinely needs to know about the offence. But negative gossip about the offender is incompatible with forgiveness.
Forgiveness is not incompatible with maintaining a certain distance from the offender if that is felt necessary or helpful, so long as that does not cause hurt to him/her.
Forgiveness is incompatible with using past forgiven offences against the offender.
Forgiveness may helpfully be maintained by being expressed (perhaps aloud) as often as necessary before God towards the offender in his absence, as well as by regular prayer for him. Where the offender is impenitent, the readiness to forgive can be expressed in this way before God. This is particularly relevant when the feelings of hurt arise. Forgiveness is unconditional and an attitude of forgiveness is not conditional on the repentance of the offender.
Forgiveness or the readiness to forgive should, however, also be conveyed directly to the offender.
Readiness to forgive is not incompatible with causing the offender to face the consequence of his sin: e.g. discipline, legal settlement, etc. (However, justice must be tempered with mercy). Nor is it incompatible with acting or praying to prevent the person doing further damage.