With the greatest respect @littledragon99 - for a woman in her late 40s you need to grow the fuck up.
You have to be reminded to pay your rent - because you have ADHD? No, you don't have to be reminded, you have to take responsibility and set up reminders to yourself on your phone, or better still, set up a standing order so that your rent is paid IN FULL AND ON TIME. How have you reached this age and not developed strategies to work with it?
You are not a good tenant. Your first month's rent was late, your second month's rent wasn't paid until it was over a month late because it was paid with your third month's rent which was also late. You admit that you have paid your rent late "every single month if i'm honest". Geez!
"She said since "I have never paid my rent on time" she needs the guarenteed airbnb income from airbnb'ing out the lounge, but this is just making my anxiety levels through the roof"
Well we know she's right, you have never paid your rent on time. You said so yourself. And the lounge was never part of your tenancy agreement, you rent your room only. And, you don't even use the lounge - you stated "when i'm at home i just collapse, i barely leave my room tbhi". The lounge is a total red herring, you're just scrabbling around for a way to not suffer the consequences of your actions.
Remember your OP? Justifying your withholding / deliberately not paying your rent? Let's look at it again.
"Recently, my landlady has begun Airbnb-ing the living room, removing the communal space I was paying for. Given this change, I believe the £1,200 rent is no longer justified."
You were NOT paying for the communal space. Your landlady kindly allowed the room to be used as communal space, but you rent only your room. You have ZERO justification to not pay your rent.
Stop claiming that anxiety / depression / ADHD make you pay your rent late. Accept that you are making your landlady's anxiety levels go through the roof. Just stop trying to weasel your way out of the shit you have caused yourself. Adult up.