All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, is His slave and Messenger.
The mother has a great status, and being dutiful and obedient to her is obligatory. Mistreating her – even with the slightest kind of mistreatment – is forbidden and it is undutifulness to her.
Being undutiful and disobedient to the mother is one of the grave major sins and results in the wrath of Allah on the doer.
Since you know the weakness in your mother, and that she is hot-tempered, then you should deal with her on this basis, and avoid being a reason for her angry.
When she becomes angry, you should stay away from her until she calms down and her anger subsides, and you would be safe from behaving with her in a wrong manner.
You have done well by asking her to forgive you, but this is not enough; rather, you must repent for having mistreated her. Your repentance should fulfil the conditions of a sincere repentance which we have highlighted in Fatwa 86527.
With regard to her being angry with you for marrying a second wife, if she has no valid Islamic reason, then you are not obliged to obey her in this regard. Also, you are not considered as being undutiful and disobedient to her for not divorcing your second wife whom she is requesting you to divorce without a valid Islamic reason.
Allah knows best.