Is it worth telling your manager that you are struggling?

It depends on the workplace, and it is reasonable to be strategic rather than either fully open or fully silent. You are generally not obliged to disclose a diagnosis. What often works better is discussing impact and adjustments — needing clearer priorities, a temporary reduction in scope, flexibility around timing — without detailing the cause. Test the water with a smaller disclosure first and see how it is handled. If the culture is genuinely unsafe, protecting yourself is legitimate; in that situation, professional support outside work and, eventually, a different role may matter more than disclosure.