Are there acts or behavior that are unforgiveable? Before you answer that, think of an inappropriate act that might be committed against your child by a family member, friend or stranger. You don't always have to forgive someone.
IMO, if the trust is broken (trust being the key word here), it would be very difficult, if not impossible to have a relationship with that person again. At the very least, the broken trust will always be a factor when dealing with the individual.
Can you forgive without condoning what happened? If you can, if you can really be honest with yourself, you can make a choice to forgive.
One thing I know for sure. Holding onto the negative thoughts, the anger, the emotions that you can't let go can be more detrimental, not only with that relationship, but other relationships, as well. Talk to a professional if these overwhelming thoughts and feelings take over your life and ability to be happy, joyful and satisfied.
Note: see 4/6/21 "Forgiveness" entry.