hi - I'm a single parent with three teenagers and feel your pain! I have to work really, really hard to keep my place just looking untidy as opposed to burgled and it's a constant struggle. It can be so soul destroying coming home to a messy chaotic house night after night and feeling too bloody tired to do anything about it Here's my tips which have helped me over the years although I'm still far from perfect - hope some at least help ; -
Declutter ruthlessly - keep chucking out stuff. The less stuff you have the easier it is to keep tidy. I found Marie Kondo helped although some of it is a bit off the wall (folded knickers anyone?). Have a google and see if it might help you
Do small bursts of things - I did read somewhere to set a timer for , say, ten minutes and do what you can in that time. You'd be surprised how much you can do in a kitchen in ten minutes to make the place look better.
Have little habits like wiping over the surfaces whilst waiting for the kettle to boil or wiping around the sink whilst brushing your teeth or putting on a few favourite songs and doing a task a song
Grit your teeth and shut the door to your teenagers' bedrooms. Let them live in their shit and concentrate your precious energy on the main living areas and your bedrooms. Just insist that they bring their plates/cups down to the kitchen and their dirty washing to a washing basket every day.
Now and again, if it is really getting on top of you, take a day off (sick or annual leave) and put some loud music on and CLEAN /declutter for a few hours and then do something just for you for a couple of hours
always try to take something from the room you are leaving to where you are going - so if you leave the lounge or the kitchen always take the cups in.
If you've got lots of stuff in the hall that belongs to the kids get them to keep them in their rooms
try to make the place smell nice - open the windows when you can, get a couple of scented candles, put some tissues soaked in fabric conditioner behind your radiators. And get your teenagers to empty the bins every day come what may
in the bathroom just try to keep bottles to a minimum - I just have one shampoo, conditioner shower gel and bubble bath on the bath . Makes cleaning much easier
as for brightening up your house - a few warm fleece blankets candles and cheap flowers make a huge difference.
on a personal level, you sound very hard on yourself. You need to fake it until you make it. Keep telling yourself that you are doing a great job and its not easy working and bringing up kids alone. If you tell yourself you are disgusting enough times you will believe it.Teenagers can be so thankless so thank yourself for being you. Buy yourself things to cheer and comfort yourself - a cheap lipstick, a bunch of daffs. Put post it notes on the inside of cupboards with sayings that resonate with you - it sounds naff but it works. Treat yourself as you would a friend who is struggling.
Try to talk to your teenagers and tell the how much the house gets you down - see if they can come up with some ideas to help. Mine are hopeless but yours might not be!
Try to put things into perspective . My neighbour lost his dd two years ago I was moaning about my lot to him recently and he said he would love his dd to be naking a mess in his home. That stopped me in my tracks