“The weak can never forgive . Forgiveness is the attribute of the strong”- Mahatma Gandhi
Firstly you need to ask what is forgiveness ?

On a general note , forgiveness means letting go of the feeling of resentment or revenge towards a person. Due to the way all humans are unique in their character and beliefs, we sometimes step on each others toes unknowingly and at that leaving one person angry and bitter . Simple things like words or actions of another person can leave a person hurt and seriously traumatized say in cases of a cheating partner , an abused child , conflict between business partners or a person who has been physically or emotionally abused, such cases would only make a person seek revenge.
While forgiveness might seem like an easy thing to speak about , it gets hard when you’re actually faced with life challenges that requires forgiveness . Also agreeing with the popular saying “ time heals all wounds ” it is also worthy to note that you’ll forgive easier when you realize and accept that No one is perfect and each person has flaws .
Everyone talks about the reasons why you should forgive , but here are the side effects of holding a grudge :

You become bitter and angry
Many people underestimate the effects of being unforgiving and are unaware that unforgiveness only takes away your peace of mind and ass such , makes you bitter and angry. Often times you carry this into every relationship and damage another person.

You dwell on the past
Holding a grudge simply means you’re still dwelling on the past and have refused to enjoy the present .

You become anxious or depressed
when you often think of that person who hurt you, you become anxious and sometimes start sinking into depression as sometimes this other person isn’ t even aware of what you’re going through and goes about living a normal life while you stay hurting .

You start feeling empty
There is no better signal for you to forgive a person than when you start feeling like your life lacks meaning or purpose or that you’ re at odds with your spiritual beliefs.

You start losing people who truly care
When you refuse to forgive , you start projecting your bitterness and anger on others and as such you start losing your valuable and well-meaning relationships with others.

https://www.nigerialog.com/family/what-happens-when-you-dont-forgive/new/#new

I concur