We sometimes forget that when someone dies, a door closes that can never be reopened. The opportunity to heal a broken relationship or say a word of encouragement or love is gone, never to return. One reason I wanted to print your letter is so others might stop and think what they need to do to keep from having the regrets you now have.
It's too late to seek your mother's forgiveness for whatever part you had in this broken relationship, or to express your love for her in spite of your difficulties. But listen: It isn't too late to seek God's forgiveness for what happened! Don't waste time trying to assign blame; you are responsible for your own failures, and you need to confess them to God and seek His forgiveness.
And God wants to forgive you, because He loves you, and He sent His only Son into the world to give His life for you. The Bible says, "But God demonstrates his own love for us in this: While we were still sinners, Christ died for us" (Romans 5:8). Confess your sins and open your heart and life to Christ today.
Then ask God to help you learn from this experience, and to share His love with others—even those who might be difficult to love. The Bible says, "Make every effort to live in peace with all men" (Hebrews 12:14).