I know that overwhelmed feeling; the way you conquer that is a simple trick but you have to really stick with it: break down everything into small tasks and small areas. Sections -- mentally focus on one small section of anything at a time, as if that's the only part of the room, or the floor or whatever, that exists right then. It's the only way to clean without freaking out at the enormity.
Instead of look at a whole room and realize you've got to tackle it all, walk to the area just to the right of the doorway, and in your mind partition it mentally into one section. Think of that wall as just one 7ft by 4ft section or whatever size, just keep it small. To you that's the only part of the room, and the things in that part, that exists right now. You have to stick with that firmly or it doesn't work.
Always work top to bottom, and left to right, it's the only way to feel like you're in control instead of being met with chaos.
Start at the top, the ceiling. Any spider webs? Get a long cloth and flip at them, or stick a vacuum nozzle on and if you can't reach to vacuum then use a broom to reach.
Now progress down a level -- any pictures on the wall? Dust the frame edges top and inner. Any furniture, a bookcase? Dust from top to bottom, left to right, meeting each physical object as the next logical task as you go.
Being systematic is the key. Always work from the top of anything to the bottom, be it a wall, a piece of furniture or a small item. And keep your tiny section in mind. Don't travel around the room randomly dusting anything, stay in your one section and go from top to bottom until everything in that section is done. Go slowly and don't rush. When I rush I get agitated and start to feel like a meltdown is possible. Going slowly in anything holds down agitation.
Leave the floor until last. Move on to the right and mentally cut off your next section of the room. Work top to bottom.
Stuff in the middle of the room can either be lumped into a section or tackled as its own area.
Do the floors last. Again think about sections. Move the vacuum across one rectangle or square as if that's the only thing that exists. Then systematically do the the next section, with a small overlap. Work around the room from the far corner to the door, but keep thinking in small rectangle or squares. A lot of people vacuum or sweep or mop "willy nilly" -- that is chaotic.
Small squares of work is the only way to tackle it without feeling overwhelmed, at least, that's the only way I can work it for me. I do it for a living and I actually enjoy the process but only if I do the "section" thing ALWAYS. Anytime I've become willy-nilly for some reason -- people in the house I'm trying to clean, something breaking my routine, it's driven me crazy. Sections sections section, and methodical left-to-right, top-to-bottom working.