To feel devastated?

I feel deeply upset.
Its my son's 9th birthday today and he is with his dad. He moved on from our long marriage just 3 months after and still with this woman nearly 2 years on.
He posted a photo of our child and his girlfriend wrote a message to her mum saying 'Thank you for his t shirt, he loves it!!'
It all felt so over familiar and breaks my heart. Anyone else think she is overstepping?