Keeping it in will only make you suffer. Therefore, let it out.

I know there are so many things that keep us busy all the time and we hardly even have time for ourselves. 

Keeping or holding someone in your hear only makes you have a hard time, so instead of keeping it all in you can go to a private place and scream or shout it out,

just let it out or you can surmon the courage to go and tell the person that you didn't like what the person did and let the person know that despite the person's actions you are ready and willing to forgive the person although I know it is something very difficult and hard to do but it wouldn't hurt to give it a try right. 

Just don't keep it in because at times the person may not even be aware he or she has done you wrong so just give it a try and also te more you keep it in you will be the one suffering or even in pain meanwhile the person will be chilling, flexing and enjoying life while it will be eating you up inside which is why it is better to just let it out or let it go.

Even the good book (The Holy Bible) which is God's word said we should forgive so in order to obey God and His word let us do as it says.