New diagnosis and he wants to leave

3 days after his ADHD diagnosis (and starting medication) my husband of 23 years told me he thinks our marriage is over.

He says our whole marriage has been a lie: that I've never known the real him and he's been acting the part of a loving husband the whole time. He says there's a block at the back of his brain that has stopped him from ever loving me as much as I love him. I don't actually believe this and nor do people who have known us well since the beginning. But it's heart breaking and frightening.

Is this reaction common? Is it the medication? How can I assure him that our marriage has been wonderful despite the challenges (which make SO much sense now he has the diagnosis) and that the good stuff we've had/have got going in a favour far outweighs the tough stuff?