Start saying 'no'...
The absolute first thing to do is to stop making the situation any worse, and the way to do this is to stop agreeing to new things.
Saying 'no' can be difficult, but there are plenty of websites out there with tips on how to do it.
A favourite of mine is "I'd love to do that, but can't right now. Could you get back to me in a couple of weeks if you still need it done?" This puts the onus on the requester to get back to me, and still shows my willingness to help.
Genuinely busy people who are asking you for things would much prefer a swift & honest 'no' instead of a 'yes' without follow-through.
Of course, the trickiest person to say 'no' to is yourself. However, the part of you that wants to do things and has great ideas is what got you into this mess, so you have to be ruthless with it. When you catch yourself starting something, say 'no'. Write down the thing you were starting and stick it in a drawer. We'll get back to it later.
The other main source of new work is things going wrong.
If your car breaks, you probably have to fix it asap. But maybe if you're really busy and can afford it you can take cabs or public transport until your workload eases up?
If the wall clock in your lounge room breaks, it'd be good to fix it, but maybe you don't have to? Probably everyone who needs to know the time has a portable time keeper anyway.
When things go wrong, you normally have to fix them. This sucks, because you have to expend precious time, energy, and attention to maintain the status quo.
Before you do this, ask yourself if the status quo is really worth all that.
You can frame it as an experiment: how about we try without a second car/lounge-room clock for a week and see if we miss it?