Dear OP, I am so sorry you have been through such stressful times. You sound like a lovely, responsible person who has temporary financial issues due to ill health.
I have had similar issues to yours. My advice might seem odd, but hear me out. I suggest you take a very basic job three evenings a week - working in a pub or restaurant. Since Brexit, there's a lot of these jobs available. All the money from this job goes to paying off your debt. It's a temporary plan. It will get you out into the world and socialising in a very superficial way which can help with depression. If it doesn't fit your social profile, tell people you are doing it as a challenge to yourself for personal reasons.
I would explain to your Dp what has happened, and reassure him and you that it is temporary. Ask if he would loan you the money to pay the overdraft off in full, then use the casual job pay to repay him regularly. If you like, stick with the job for a few months to rebuild a bit of extra cash.
You might want to consider insurance against illness for the future though they may not allow depression as an existing condition.
Meanwhile, if you have long-standing clients, I'd be honest. Write a professional letter to say that you fell behind with work this year due to a sudden and acute illness (depression is an illness - do not add guilt or shame to your existing pain load). You tried to keep on top but fell behind. You are now recovering and returning to work.
Schedule a gradual return. Take on projects that are easy or quick or with clients you trust to build up your energy and confidence.
And of course schedule a lot of self-care - physical exercise, some sort of mental support through meditation, journalling, counselling etc.
I really hope you recover soon. You have been through such a sad time.