Her bratty adult kids are ruining our relationship

I'M A man in my 50s and have been seeing my partner for two years.

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We both have grownup kids from our previous marriages but, while mine are easygoing and supportive of our relationship, hers are difficult.

She also frequently cancels on me last minute because of issues with her daughters.

I've been very patient but I'm reaching the end of the line. I planned a special birthday celebration at her favourite restaurant last week and she called half an hour before to cancel because her daughter decided she didn't want to go.

Meanwhile, I'm sat there with my son and daughter with a gift and balloons, looking like a chump.