It is a lot easier to correct a sloping floor on the ground floor, then the first floor. Normally on the first floor sometimes you would have to take the ceiling down pending on the joists if you would need to change them, sometimes you can just remove the skirting boards, floorboards and correct the problem from the top, you might have a little crack in the ceiling, and sometimes people can have accident and break through. Most of the time is to price the job and hard work.

there's a few things that could be wrong... it could be a wroten beam that the joists run into that is slowly degrading... it could also be as you suspected the wall beneath was load bearing and the floor has dropped as a result of that. one way to try and figure out the problem is try moving over the floor and listen for movement if it's still moving then you might have a large problem however if its sturdy and there isn't any you could simply level the floor out using fillets and new t and g. Best way is to get a joiner to come a good one should be able to know what's wrong just by the feel and look of it. hope this helps

If it’s a sloping floor that means joists are sagging. Meaning that joist aren’t big enough in profile to take weight of floor !

It could be a number of different things without removing the floor boards and having a look it's really impossible to say what the best solution maybe while sagging joist maybe one reason it could also have been built that way, it may even be subsistence the only thing I can assure you it will not be a cheap fix. As everything above and possibly everything attached below may need removed along with the joists themselves.
Good luck Alex