One reframe worth having, because this will happen again and the guilt makes it worse.
A break is not a failure of discipline and the restart cost is normal. Everybody doing this around a job has three-week gaps. The people who look consistent have simply got the re-entry cost down, not eliminated the gaps.
Two things that reduce it structurally:
Keep the project small enough to hold in your head. A large tangled project has a high re-entry cost permanently. Every simplification you make is also a reduction in how hard it is to come back.
Have something ambient going on. Answering a user email, reading the analytics, a five minute thing on a Sunday. It keeps a thread connected and means you are never fully cold.
And a genuine question worth asking after three weeks of not opening it: is this a re-entry problem or are you finished with it? Both feel identical from the inside. The test is the tiny-task evening in the first answer: do the twenty minutes, and notice whether you want to keep going or whether you are relieved when it is over. That answer is worth knowing either way, and you cannot get it by thinking about it in the evening.