A person that my late husband had great regard for was Viktor Frankl. He was an Austrian neurologist and psychiatrist as well as a Holocaust survivor. Please research this individual and read his book. You may also watch this short TED talk, the English is not really good but good enough to understand what he has to say.
If you cannot go to your mother’s funeral because it will hurt you in a way that will endanger your stable self then don’t go, what is important is your own mental and emotional stability because if you are not stable then others will suffer too. It is not selfish to take care of oneself, it is necessary for the sake of others.
Mother Theresa said to her sisters that they had to take care of themselves if they wanted to take care of others (they had to sleep and they had to eat).
I hope that what I am telling you will help. Be the best that you can be regardless of the shortcoming of your mother and others.