How to let go of grief and not hold a grudge?

I'm sorry you lost your grandma a few years ago.  It's a difficult loss and I hope you have had support from friends and family.  Grief comes from losing someone you love and it's a process that takes time.  Typically, the first steps is acknowledging your feelings and emotions and that those feelings may come and go in waves. With processing grief you may find yourself denying the person has passed, being angry at the person for leaving or angry at other person about the death. 

It's normal to want to reverse time in a way, to bring them back, to make changes to prevent the death.  As you begin to accept your grandmother's passing, you may notice you feel more emotionally down and this is also normal.  It's important to be aware if the processing of grief progresses to depression, and reaching out for more support if it does.  Each of us grieve differently.  Seeking out support from those who love you, supporting yourself emotionally and being kind to yourself as you process through each stage of grief is important.  Be gentle with yourself as you walk through each of these stages of grief.  There are many resources available to support you: BetterHelp counseling, SAMHSA (800) 662-4357, and National Crisis Hotline (800) 273-8255.

Now onto your other question, it sounds like you may have regrets and those feelings have progressed to holding a grudge.  Sometimes when we hold grudges, it comes from wanting acknowledgement that the person who hurt us realizes they wronged us and because of our grudge they will change their behavior.  But in reality, that other person is going on with their life and you're stuck in those feelings.  It has the inverse effect you want it to. 

It's also sometimes easier to blame someone else for a simple decision that you made.  Sometimes we have to work through our own forgiveness for decisions we have made and that's beneficial work too.  It may also be helpful to write down your regrets and look at them with a critical eye asking factual questions to see if they are based more on feelings or reality.