How do I convince or get my grandmas trust back?

Try to explain to her that you really like 'Bob' and in order for you two to have a future together you need to get to know his parents better. Tell her that you will go over there at a decent hour and come home at a decent hour. For example from 5PM-8PM or whatever. Don't do anything ual with 'Bob' again until your a bit older, maybe she will trust you more. See if you can invite 'Bob's' parents over for dinner for your grandparents to meet them. Tell her that you are willing to do anything to regain her trust and you have grown up and realized your mistakes but you do not want to ruin your relationship with 'Bob'. Good luck..