The pomodoro process works for me and for a lot of people. Just set a timer for 25 or 30 minutes and make a rule you can't do anything else until the time is up. Just sit there and do nothing if your brain refuses to cooperate. Eventually boredom will do the trick.
Finally, creative work often involves lots of bits that aren't really creative. Source control, editing, issue trackers, etc. You can gain a lot by setting up a strict, thoughtless way of going through these motions. Makes it painless and not a hurdle, eventually, even if it still eats time.
Now that I don't have to think about lunch, I can spend that mental energy thinking about other things. With enough ritual and routine, I can even dedicate that mental energy while I do those things.
Ritual and routine don't stifle creativity, they give it the space to function.
Many writers talk about routine being critical to drive their production.
No amount of routine or pomodoro or any other system has overcome this, my brain simply feels wrung dry of any more creative ability.
It's like that other adage about artists and critics. I'm paraphrasing here: "Critics will get together and talk about meaning and form and style. Artists will get together and talk about where to get cheap turpentine."
It's almost you don't worry about the actual creative part of things. Just do things. Most of it will be bad. If you're counting on only creating good things or let creating bad things discourage you, you'll never actually create.
Maybe you do feel wrung out after a few hours, but maybe if you honestly kept at it for another hour, you'll get a second burst. Maybe not. Maybe it happens sometimes, maybe not others. The real point is that unless you try it every day, you'll never find out.