Perhaps his anxiety had something to do with the fact that another singer was just gunned down? I really think we need to be more sensitive given recent events.
Watch his next album, if he ever makes one, flop. I feel for him but if you keep canceling events like this, it will definitely do damage to his career and image. He will have trouble booking venues as well if people think he will cancel and lose them money.
Having anxiety issues aren't the problem. The problem is that; it's clear, by now, that he has them and he's not doing anything about it. There's no easy way to get over it, no, but what he could do is not sign up for these events - in case the anxiety strikes, and it'll save fans being disappointed. I don't think this is the first thing he's cancelled this year didn't he cancel some promo appearances at the start of the year when his single came out? I can only imagine him announcing a tour and then cancelling the day before when people can't get refunds and had booked travel etc.
It's sad. Get well soon Zayn. You're right to think of your health first, but also think of your fans when planning events in the future.