I hear you. Learn from me. I am now in my 50's
Some people will NEVER change. They either can't or do not want to change, but we have to learn to just go on either with them, or by ignoring them..
I have known my husband since we were 13. His mother, my MIL, has treated my wonderful husband, her son as though he is the 3rd person in line for her heart. .
Can you imagine? We tried as you are doing.. Giving a heads up, including her in the plans, phone calls, notes, emails, offering to pick her up.. Nothing. It is hit and miss. mostly miss.. And yet she cannot bend backwards far enough for her daughter, my husbands sister.
The difference? My sister in law married into money..
My husband even went to therapy with his mother. This is were she stated, "Mothers are always closer to their daughters".. The Therapist told her, "no that is actually not true, and besides it is your choice to have a favorite, most moms never admit they have a favorite". This not only infuriated me, but broke my heart. And yet we continued to try to include her for years.
What finally broke the camels back is when our daughter was about to graduate from High School. In all of the school years my MIL NEVER attended 1 event our daughter was a part of. We all live within 20 minutes of each other. And yet she went to every event, that her grandsons participated in.
I decided, this had gone too far. I no longer keep her updated, informed, or invite her to anything. I no longer remind my husband about her birthday, anniversaries. I do not purchase gifts..
These last 5 years have been bliss. I do not worry, fret, or concern myself.. She has learned, without me.. Her son and her grand daughter is hit or miss. Sometimes they remember to include her or informe her.. but usually not.
I now give my attention, my love and my time to those in my life that are kind, interested and want me to be a part of their lives.
Being related by blood, does not make you family.. Sometimes it is good friends that are your real family.
Find a way to come to peace with this. If you need to speak with a professional, do it. You are worth it. You deserve to be loved.