help with social advice

while making a new friend i said "omg yes its ok you can sit with me" without thinking when she asked if she could. i agreed cause im over the top when meeting new people and i like to be nice...

how do i get out of this i, dont have any issue with her i just want to be alone, that painting class is like my sacred alone time during the other greuling and exhausting classes i take and socialise in.

i have adhd that i was just recently diagnosed with if that explains anything, either way im just having a really shit time in highschool rn due to my anxiety and i would like to keep to myself.

pls just give me advice that doesnt make me sound like a bitch and doesnt look like a stupid excuse. (i would rather not mention the adhd and anxiety to her as im not familiar with her at all)

this sounds like a first world issue but i dont fuck around with my alone time in painting class.. im handling business