Firstly, I love your striped wallpaper! A great choice for that end of the room.

It's hard to tell from the photos if this would work, but have you tried turning the couch through 90 degrees? Yes, the back of the couch would be facing the room but I think maybe this would break up the space a little and make that end of the room feel more cosy. Some pale scatter cushions would help it feel brighter.

The big square shelves between the kitchen and dining room- could they be moved somewhere else, maybe to another room? They are really too deep and dark for that space and it looks like you could display those items on narrower shelves.

With the shelves gone, you might be able to turn the table 90 degrees as well. Then it could be spaced away from the window. Yes it takes up a lot of space in the centre of the room, but I think it's best to work with it instead of trying to push it as far away as possible. Pushing furniture against the wall generally makes a space feel smaller, not bigger.

I know you are on a budget, but would it be possible to replace your curtains for something lighter? They are making the room feel much narrower. Pale, floaty curtains would make the whole space feel brighter - though I know this won't work if that room is cold in winter.

I would add small photos or pictures with bold colours in dark frames to the wall that separates the kitchen - try two or three beneath each other. Make sure they aren't covered when the door opens.

Lastly, I think the end tables and lamp right inside the door feel a little big for the space. Try replacing them with a taller, narrower table and a smaller lamp - maybe you could use them elsewhere in the flat instead.