I'm a terrible person for what I did, and I will never get over my guilt
Is it really that bad? What did you do to cause your feelings of guilt?
I missed my grandfather's funeral.
That's something to feel a little guilty about, sure.  But it's in the past!  Learn from this mistake you feel bad about and avoid repeating it in the future.
I can't do that. What did I do?
It's normal to feel guilty about missing a loved one's funeral. Over time I think these feelings will fade. Talk to your parents and see if they can help you out.