Yes in my opinion it is, as he is investing time into this relationship at the detriment of you and your mental health.
But I honestly think you know the tip of the iceberg with these two.
He is still playing a selfish and entitled game with you, minimising his behaviour and now claiming to ‘regret it’. He can’t claim that when he’s been carrying this sneaking around out for 8 years!!!!
I know and realise you’re in shock but seriously, you’ve known this woman has been lurking for all this time. It’s time to draw a serious line in the sand.
For many that would be looking at separation and divorce. For others making it clear that any contact with this woman is unacceptable but tbh I think your husband will just go deeper underground.
You do realise you matter, your pain, your boundaries, your emotions matter. He has made it clear they do not matter to him over his need for ego kibbles.
That is not ok! It’s time to be your own hero, your own best friend. And I second reading Glasses book, it will open your eyes.