In our lives we all have had disappointments and out and out experiences with another person that nags you so much because you haven't been able to forgive them. The Bible says we must forgive our tresspasses and those who tresspass against us. I find that hard. Can you do it? What helped?
Holding grudges is normal but only for a very limited amount of time, the length of which is usually relatively short. Holding long term grudges is stupid as it's psychologically and emotionally toxic to one's well being and they can develop a life of their own.
Mentally I know that we are all imperfect, that we will let someone down, or cross someone eventually and given that should forgive others their humanity as we would hope they forgive us ours. Just remember none of us are perfect and no one can or will live up to all of our expectations, even ourselves. Forgiveness has more to do with our mental and emotional well being than anything else, hate and anger can be corruptive over time, twisting and mal-forming our overall outlook on life. Forgive so you can move on, if they don't then that's on them.
Oh and it's "forgive us our sins
as we forgive those who trespass against us".