Ifyour here's something for you.
I was born in England and lived there until I was three. My entire family is English. I consider myself Welsh as it is all I have ever known.
If you can't hear someone talking about the Welsh not without becoming so irrationally angry and taking it so personally, I think the issue lies with you rather than with the people.
As for 'it would never be tolerated the other way around', of course. Because we never get called sheep shaggers, or told to let our language die, or bullied for talking joskin, or accused of changing language to make someone else feel bad.
We never learned any dubious facts about the world war, not sure where you got that from. We learned about Tryweryn because its a part of Welsh history, and never in the sense of 'Look what these evil English did', in the sense of this is what happened, this is why the lake is there, this is the politics of it all.
Should Germany not be shown as the antagonists of world war 2? Why can't England be proud of its history without having a go at the Germans? Feel free to accuse me of strawmanning or whatever but its the same thing.
I've seen children bullied for speaking Welsh in the playground. Teased for taking part in eisteddfodau. The situation at my uni is ridiculous, people are openly criticised for daring to speak Welsh.
The Welsh language has suffered massively because of English politics. Not average Joe English people, and not today, but to deny it happened, or to take offence at it and say we shouldn't say it, is an attempt to whitewash the history of wales.
I have never met more friendly, proud and welcoming people as I've met in Wales.