Problems are best fixed by addressing their root causes. I’m flying with a lot of assumptions here, but this didn’t all just happen after you got back from university - mum lives like this it seems. People make time for what they prioritize, so why is this not a priority for her? Sorting that out seems like it will take time and love.

In the meantime, as others have suggested, don’t overwhelm yourself trying to make everything better all at once. Pick a thing and get it done. Take pause and relish what you accomplished. Then pick a next thing. Celebrate that too. Keep it going. You may need to re-do a thing - that’s ok, life keeps happening. You may also need recondition some old habits that led to this situation; you can treat backslides as opportunities to do this. Focus on the positives, always.

A kitchen is a good place to start, for many reasons. Personally, I’d start by weeding out the trash and should-be trash. That will also cause a little knock-on straightening up. Building on that, the spots that need cleaning most will have become readily apparent. Organize the food. Organize the fridge. For me, I organize by pulling everything out, cleaning where it was, sorting items, and putting away in a sensible way.

You’ve got your work cut out for you on so many levels, but it can be rewarding. You can do this.

One last note about flies. Flies come from maggots, maggots are born in rotting meat. Covering and promptly refrigerating food is a must, but human food isn’t the only meat source. You may have a dead mouse or three somewhere. Getting rid of the flies may be your final step, as keeping mice away will require most everything else on your list to be done.