Life happened. Work, illness, a holiday, or the ordinary business of being tired, and the training quietly stopped 3 weeks ago.
The guilt is doing you no favours here. Nothing you built has been erased, and a comeback is a smaller job than a start.
What happened while you were off
Two things, and neither is dramatic. Your dog kept practising whatever the environment paid them for, and you stopped paying the things you wanted.
If your dog is in early adolescence, something else may be layered on. Dogs show reduced responsiveness to their caregiver's cues around the onset of adolescence, and that responsiveness returns later in the stage.
That is developmental, and it was not caused by your 3 weeks off. It also does not make waiting the plan.
Pick 1 skill, and only 1
The instinct is to restart everything at once, which is why most comebacks last 4 days. Choose the skill causing the most trouble this week.
Name it plainly: Recall, Loose leash, Settle, Greetings, Impulse control or Alone time.
Write it on the fridge, and ignore the other 5 for 2 weeks.
Decide what paying it looks like, and put food back in your pocket tomorrow morning.
Drop the criteria to where your dog wins
Start indoors, with no audience, on the version your dog could do at 6 months old.
Halve the distance and take the distraction to nothing. Recall from 6 feet in the hallway, not 60 feet at the park.
Run 5 minutes, once, tonight. One 5 minute session you actually do beats the 30 minute session you plan.
Add 1 harder element at a time: distance, then movement, then somewhere new.
Make it survive the second week
Behaviour change is built from management plus reinforcement, and a comeback is mostly the reinforcement half. Reward-based training was also associated with higher reported responsiveness and fewer reported problem behaviours in survey work.
Attach the session to something you already do daily, like the kettle or the evening feed.
Put the long line back on outdoors for 2 weeks, so the outdoor version is not eroding while you rebuild indoors.
Mark 14 days on a calendar, 1 mark per session. Miss a day and mark the next one, without starting over.
Safety note
If the gap contained something bigger than a lost habit, treat that as its own problem. New fear, growling, biting or a sudden change in behaviour needs a veterinary examination first, then a qualified behaviour professional.
Today's version, in one line: 1 skill on the fridge, and 5 minutes in the hallway before bed.