It's not my party so I can't cry if I want to

[bluemarmite]

He's been separated from his wife for more than 5 years (she left him long before we met). We've been seeing each other for 2 years. We both have children.
His (ex) wife refuses to meet me.

There's a big family gathering for their son's birthday and and me and my child are not invited because she (wife) is going.

I understand that it is their child's celebration and actually, if she doesn't want to meet me, ever, that is up to her.
But, I'm uncomfortable about the conflicted position it puts their children in - they spend time being nice with me, but then when they are with their mum, I'm clearly considered to be unmentionable.

Also, the aibu bit, being excluded is making me feel quite insignificant. Am I?

YANBU and I don't understand why she should have that attitude to you or why your dp gives in to it. You and your child are a part of his life and their dc life.

Does she have a new partner? Does your dp meet him and is he invited to this party?

She is seeing someone else, but she would rather he didn't go so that I wouldn't either.
It doesn't make me feel much of a part of their life to be honest, but, being sensible, it's not about me, it's about the birthday boy and him enjoying his birthday rather than having an upset mother.

Can you do another party with him with you’re dp?

Yes it's about the birthday boy but I don't understand why his mother should be upset when she ended the relationship. What exactly upsets her about you? Seems a bit rough on her partner to be excluded as well so that she can justify excluding you - it's all pretty unecessary drama.