I completely sympathise with you.
You had good intentions for your friend and took a hit in the long run.
You do sound like you're struggling a lot, but hopefully you can find a solution as others have suggested.
You don't need to take the world on your shoulders and your friend, although a good friend, isn't your responsibility.
Many lose care of their own world when they care for others - yet it doesn't mean we shouldn't look out for them. But when it affects your health, sometimes taking a step back is needed.
Although I don't have the solution as I don't have a mortgage and I don't understand these things, all I can suggest is time out to yourself to relax, remember to self care (I know it's hard when we're in a funk), but even something small each day and remember you are worthy of a good life and you will find a solution.
My favourite motto is - Nothing is linear.
It's very true. This will soon pass, but fundamentally, look after yourself more in the process and find ways to release the stress. It could be gentle exercise, herbal tea, relaxing music, breathing exercises and telling yourself you will get through it. Encourage yourself and love yourself.
I know this sounds cliché, but it really does work.
Sometimes writing a diary about how we feel each day and analysing it "ok, so why do I feel this way and what can I do to solve it?" Don't worry if you can't find a solution, it will come to you in time.
Putting yourself first when you feel like this is fundamental to everything.
Your friend should manage, especially if they're struggling financially - they could go on UC with the housing benefit element if they don't have enough money. It pays quite a fair bit of rent for them, or go in to a house share etc..
If you get the house sold, that would be wonderful and a huge stress off your shoulders.
You sound like a very kind person who cares about others and that's a beautiful thing to be. But ultimately, take care of yourself when you see things slipping as we can't look after anyone if we ourselves are struggling
I wish you all the best.
Nothing is linear, this will soon pass.