Advice needed

My partners mother does not like me. A little background. I am a single mother, I have two children from a previous marriage. I met my boyfriend 3 years ago and we have been together ever since. We live together, my children adore him and he adores them. The only issue we have is his mother does not like me or that I have children. He has no children.  His parents are divorced and his father loves us, we all make plans often ect. His mother won't include me and my children in anything. At least once a week she gives him a hard time because he doesn't call her enough. See her enough. Ect which is nonsense.  They always dinner or lunch once a week, which is great I want him to spend time with his mother. But she acts like we don't exist. It's hurtful. I have only seen her maybe 5 times. Each time I am polite I ask her how she is and try to make small talk she is usually cold and short with me.  She goes out of her way to be rude to me.
for example we were sat next to eachother at one of my partners events and while I was away from my seat she took my belongings and moved my seat. I was appalled, I told my partner and his response is that's how she is and nothing will change her. It saddens me, I wish she would give me a chance but I'm also angry as she is absolutely rude.  My family welcomed him immediately.  I don't understand why she is so rude. She calls, she barks, he runs to appease her. I have explained how I feel to him numerous times and he just sticks with he general response, that's how she is. Sorry this goes in circles. Any advice?