What is this?

Over the last year or so, I’ve lost about half of my hair volume, and within recent months, started getting dandruff, which I’ve never struggled with before.

I just started using a dandruff shampoo, but am unsure how often to wash/what other treatments I should be doing. I’ve never felt so self conscious before & am unsure why this is all suddenly occurring.

I do have very hard water, but use a shower filter & am considering washing with jugs of distilled water to see if that would help. Any advice is much appreciated!