I'm currently on a "rationality as 'skills you practice'" kick. I'm really into subtle cognitive skills. I do think they eventually pay off.

But, realistically, if you have a major problem in your life, my experience is that the biggest effect sizes come from radically changing your environment.

Move in with new roommates.

Get a house in a new neighborhood.

Get a new romantic partner.

Get a new job.

Move onto a new team at your current job.

I'd count "install a serious internet-blocker on your computer that kicks in automatically at regular intervals" as an "environment change", since we do live our lives largely "on the internet."

You want your general life situation to feel like flowing downhill, not having to trudge uphill. Often, lots of little things about your physical and social environment make things frictiony, or demotivating, 

When I briefly lived on a boat where going to the bathroom required completing a minor obstacle course, I was surprised how much I just automatically got enough exercise and lost weight. I haven't succeeded at finding a way to live longterm with those properties, but, the proof of concept had a pretty enormous effect size.

If you feel like your life is sorta stuck, you aren't very happy, or productive and it all feels sorta shitty and intractable, consider rerolling some major aspect of your environment. This is pretty effortful, and if your life sorta sucks it may be challenging. But, I think it's more likely to work that a long slog of "try to get an exercise habit, try to get incrementally better at focused work."

You can change environment in an intentional way ("On reflection, my job does just suck. Or, something specific sucks about my team, let's go talk to my manager and try to switch teams."). But, anything that radically rerolls your environment has at least a decent chance of rejiggering whatever was making things suck, even if you don't have a good model of what was wrong.

(My second biggest piece of advice, btw, is "check if you're depressed, and get medication for that")