To start, determine for yourself the cause of your resentment. If it comes from your idea of what mommy did for you, something that you "should" or "must", the problem lies in you. No man nothing in the world should not have another, even if we are talking about a mother and her children. We all live according to the laws of goodwill, and your mother also needs you as you are to her.
If the reason of your resentment lies deeper, think about her mother as a Mature man, who knows everything and knows how. Think of her as a child, which is not always properly educated. Everything begins in childhood, and it is possible that the reasons for her behavior are there. But the baby's not his fault that he was raised that way, and mom is not to blame for those aspects of her character that led to your conflict. Try to put yourself in the shoes of a mom, feel her position. It is possible that you will see the situation entirely from a different angle.
To forgive mom, write on paper all the things that you care about. Write in detail, in color, in images. Leave a piece of paper all the things that you care about. After you feel out on paper really all solemnly burn it. You will immediately feel relief.
If you feel that something is left in the shower, close your eyes and imagine all the things that you like everything you love the world. Imagine doing my favorite thing, and then tell yourself that without my mom none of this would be. Mentally ask for forgiveness from the mother for what had offended her.