That is the nature of depression OP, as you know. It distorts your thinking and your reason. You have to challenge your thoughts, investigate them like a detective. We have hundreds of thoughts a day and most of them are plain rubbish. Just because we have a thought, does not make that thought accurate. You are not at the edge of a dark hole. You are at the beginning of a relapse, you already recognise it and you are looking for ways to come out of it.
Three weeks away is not acceptable. Are you able to phone first thing for cancellations each day or can you request a telephone appointment? You need to dig your heels in and insist on help sooner. Antidepressants that are doing their job mean you should not have relapses. This is fixable.
You felt better having someone around. Can you think of ways to be around people each day for a little while? A part of coping with depression is reaching out to your friends and family. Tell them you may be having a little relapse and ask for their support. You don't have to fight depression on your own. Ask for help. That should be another tool in your box of coping mechanisms and if it isn't, make it one. Tell them you want to hear about their day, you want to hear about their problems, and steer away from discussing depression too much. A little yes, but too much can be counterproductive and leave you dwelling on how you are feeling.
I strongly recommend getting out of bed straight away in the morning. No it's, no buts, you wake up, you get up. Straight into the shower, even if it makes you cry. Dressed from shoes to head, hair and make up done. Then breakfast, even if it is just breakfast biscuits. Something to level your blood sugar. Then out of the house for a minimum of a half hour walk. You could do all these things when your depression was under control and you can do them when you are starting to dip too. Get tough with yourself.
I know the terror of going back to that place when in the grip of depression. I was inconsolable when I first relapsed. But with every relapse, you learn to recognise them quicker, you learn more and more coping mechanisms. Get seen quicker. Even if you have to go to out of hours.