I've made the same mistake as you. I decided to start trading a while instead of keeping holding as I did earlier. I've sold a part of my wallet at $35k and that night BTC reached ATH at $39k 
I figured out that I need to point a mark for me when I'll be enjoyed to cash out. I bought it when the price was $32k and I would be happy to make 400% of profit so now I will wait and hold until that moment. You can do as well
This is a very common occurrence as bitcoin always defies our expectations, you sell thinking the top has been reached only for the price of bitcoin to keep going up, however I still think that you sold at a good enough point. I figured out that I need to point a mark for me when I'll be enjoyed to cash out. I bought it when the price was $32k and I would be happy to make 400% of profit so now I will wait and hold until that moment. You can do as well
Since then the price has experimented two corrections that have sent bitcoin close to the 30k level and now we are at 32k so in fact you could use this dip to buy more bitcoin for a reasonable price if you believe the price is going to keep rising in the future, but if you do not believe that is going to be the case then the best move is to keep holding your fiat or stable coins.