With kindness OP, you will find that your employer's 'full salary sick leave' policy is discretionary. Given that you've only been there for 10 months, I expect you'll find out sooner rather than later if you keep taking big chunks of time off.
I can totally understand wanting to be at home for a therapy session, been there myself. But wanting to do 'mindful, wholesome things' isn't a particularly good reason to take the rest of the week as sick leave.
As someone who once had to take six months off work due to anxiety, I understand the stigma around it. But I also know that it's unlikely you'll feel much different in a week's time, and the longer you put off going back to work the harder it will be to convince yourself that you don't need 'just another week'.
If you are genuinely too unwell to function in work, or going to work is likely to make your condition worse, then of course you shouldn't go in. Your GP will have no issue signing you off for longer if they agree, and can help you work towards a return date with your therapist and employer.
Be aware though, three weeks is a lot of time for anybody to take off in their first ten months. It wouldn't be unreasonable if your employer to worry about this becoming a pattern, especially if it's impacting your colleagues.