There is no objective reason for you to be heartbroken.

The concert hall is a public venue and as such, doesn’t belong to anyone. Her daughter happened to perform there, so of course her mom would attend. Does she need any other ‘motive’? It really doesn’t matter who she brings with her. That’s her personal life. You are assuming who the guy was, taking it as some sort of personal(?) attack on you by invading your ‘home’ when it was all circumstantial.

You keep holding on to this relationship not because it is helping you concretely with your issues (whatever your issues are) but because of an emotional connection with her - and one which you’ve built up in your head to disproportionate levels. You know this already and if i recall she stated this explicitly herself but: she doesn’t have a personal relationship with you - she is not your mother, nor your friend and doesn’t owe you any explanation on how she spends her free time.

I probably should know better than to write this but it just had to be said.