I have fallen behind and am being murdered by overtime at work. So, I have decided sod overtime, I need a life - they can hire agency staff and I can do what I want to. Which is learn Welsh.
However, how do I get back into the habit. Do I have to go back to the start!?
The general advice usually given is to start from about two lessons behind where you were last time. You’ll be surprised at how much you remember. If after 5 minutes or so it all seems very alien to you try going back another 2 lessons. You shouldn’t have to start from scratch again unless you only reached lesson 3 last time. 
Yup, as Geraint says, don’t go too far back - if you end up certain that you need to after trying a couple of refresher lessons, it’ll still be there - but it’s more likely that you’ll get through the pain barrier and realise that you’re most of the way back up to speed in the first 30 to 40 minutes… 