It would be incredible if you could start training your dog and every day they would improve and then you would reach your goal and that would be it. Frankly, it would make my job and my life with Mars a lot easier, but it isn't realistic. There's always going to be setbacks, sometimes these are just a Bad Day and your dog bounces back the next morning, sometimes they're full regressions and your dog acts like they've never trained a day in their life. It is important to remember that this is completely normal, it's not your fault and it happens to everyone during their training journey.
I could fill a book with things that could cause a training setback. Off the top of my head:
Teething. Mars regressed massively because his teeth hurt too much to take food and when his teeth were better he was impossible
Adolescence. Again, Mars. Dogs aged 6-18 months can be unpredictable while they grow and mature. Often they grow in confidence which leads to them pushing boundaries.
Injuries/Illness. Dogs that need crate rest for a long period to recover from something. Dogs acting out because of a pain like an ear infection. Dogs with a tummy bug or kennel cough. Just like we feel off when we're unwell, so do dogs.
Females in season. I'm also including phantom pregnancies and mastitis under this umbrella but they could all go under injuries/illness.
Off-lead dogs. This is a big one for reactive dogs or recall training, other people being irresponsible can unfortunately set back your training in a big way.
Holidays or travel. A big change to their training routine such as you going on holiday or a work trip can affect their progress, largely dependent on who they are left with.
So how do you fix it? Fixing it is actually easy because all you're going to do is go right back to square one. Firstly though, I'm either going to recommend you do one of two things:
A day at home for both of you to rest and take a break
This is for you if you are feeling completely burnt out and de-motivated. If the thought of putting boots on and going back outside makes you feel frustrated and irritable you need a rest day.
Get back out there ASAP so you don't internalise any anxiety
This is for you if your dog was rushed by an off-lead dog or attacked and you are feeling anxious and panicked. If the thought of going on a walk make you feel scared of it happening again you need to get back out and shake it off.
Next we're going to get back to square one. Whatever you did the first week you began training, maybe your trainer sent you a training plan to follow, do that. This is going to help both you and your dog because it's going to be easy, you've already done it before and you've likely improved a lot since then too. Typically when you're training, you're constantly advancing and trying to improve, so you're working in a constant state of increased difficulty. Taking a week to do the easy foundations from the beginning will boost confidence for both of you and will allow you to spend some nice, happy and engaging, moments with your dog- which is what training should really be about. It's also a good way for you to see how far you've come, things that you struggled with the first week will now be second nature.
This doesn't mean you have to redo your entire training plan from the beginning, we're just going back to basics for a week and then you can hop right back to where you were. Your dog won't have forgotten all the work you've put in previously, they just needed a week to rebuild their confidence and strengthen the foundations. Think of it like studying Maths at school. Most lessons you'd learn something new but sometimes you'd be having a bad day and you'd struggle in the lesson and you'd forget how to do things you used to be great at. Now imagine if your teacher responded to your frustration by spending a couple of days giving you lessons from a month ago, stuff that you know like the back of your hand. It gives you time to reset and rebuild your confidence.
The most important thing is not to panic. This happens to everyone, professional trainers included. It doesn't mean it's your fault and likely isn't anything you've done. Take a break, reset and remember that training is supposed to be fun. All of this might even get you to step back and readjust your training plan so it works better for you and your dog, if you'd got stuck in a routine that was causing stress or frustration.