I've read both your threads, Garlic, but not posted until now. I'm sorry that it has come to this but I think that you knew it was going to right from Monday, when you started to get that anxious feeling about it all. I've been in that situation as well, and know the dread of "this is going to end".
I think that you will find it difficult to live together fully amicably, it's going to be awkward without ground rules - so I strongly suggest you lay some down, including a rota of things.
The rota will include who gets the car when - if it needs to be flexible, then work out how, so that you both get fair shares on it.
You should cook separately, and wash up your own things only. You should also each do your own laundry. You're effectively flat mates now, not partners, and your behaviour should reflect that. It might be ok to cook together occasionally (I used to with my lodgers sometimes) but do not make it a regular thing.
Do your own shopping,
Have your own shelves in the fridge and freezer. Label your stuff if necessary!
Rota for cleaning - there's no reason why you should do it all now!
And your bedrooms should each be off-limits to the other.
It might sound petty, but it really is the only way to make it clear that this is now you two, being separate, in the same flat. Where it will get more "interesting" (and probably highly uncomfortable) is when each of you goes out, or has someone back to the flat - you'll need rules around that too.
Good luck - I do feel for you 
