The barrier is re-entry cost, not motivation, and the fix is to make the first session trivially small rather than to find more energy.
What works:
Open it and do one tiny thing. Fix a typo. Update a dependency. Rename a variable that has annoyed you. Twenty minutes, deliberately below the level where you have to remember the architecture.
The point is not the work. It is that opening the project stops being a decision, and the second session is much easier than the first.
Explicitly forbid yourself from doing the real thing on the first evening. The reason you keep deciding not to start is that starting means confronting where you were, which is genuinely a large task after three weeks. Remove that requirement and the barrier collapses.
Read before you write. A session spent just reading your own code and remembering how it works is a legitimate session. Nobody counts it and it does most of the work.
Do it at the time you would normally, so the slot survives.
Three or four small evenings and you will be back to normal. Trying to leap straight back to a full session is what has kept you out for three weeks.