As you have seen, it's very hard to re-establish momentum when lost—even when breaking things down to small chunks.

The issue is self-editing / perfectionism. You need even smaller (trivial) tasks that you can do without self-editing, that will lead you to do the smaller tasks with a sense of urgency, so perfectionism doesn't take over.

One way is to run a timer that tracks how much you work. So you turn the timer on when you start and off when you stop, and you track how many hours you spend actually working.

When you come back to work, start the timer running. That is your ritual. Now, if you start procrastinating (i.e. surfing), the timer is running in front of you and you will start feeling guilt/a sense of urgency because you are "cheating". You will then get to a point of unconfortability where you will have to start working.