Variance is a natural thing in statistics and many people don't realize just how much ratings can naturally fluctuate in chess. This very well might be nothing that you've done. It might just be normal ups and downs in rating. Just realize that rating is NOT equal to your ability. It isn't like you are "forgetting" what it took to be 1300; it just so happens that your recent performances have put you under 1300 rating for the time being.
It also doesn't help that many people like to be vocal about their rating "ups" and accomplishments (I like hearing these and motivating others), but hardly anyone discusses their "downs" and short-term setbacks. This creates a misleading illusion that everyone works hard on their chess and steadily gains rating over time. Actually, this would be very suspicious if this was the rating progression because everyone I've seen has MANY ups and downs in rating along the way.
Without knowing your specific case, my advice would be to put more emphasis on learning and gaining experience. If you put more focus into the learning process and don't worry about rating too much, then you'll be getting better as you learn and improve your overall chess ability and indirectly, your rating will catch up with you as you go up 
About one year ago, I had a significant "down" moment in my chess rating, despite me REALLY trying hard to reach my rating goal(s). Well, this blog post describes how I felt and then a month later, I had recovered (second blog post). I can't tell you how many ups and downs my rating has had over the years though...and this is normal to everyone; it is just that this is hardly ever talked about!
https://www.chess.com/blog/KeSetoKaiba/dropped-more-chess-rating ("down moment" for me)
https://www.chess.com/blog/KeSetoKaiba/today-is-the-day-2000-rapid (one month later, but other times it takes much longer to recover)
I can't understand why I've lost around 300 rating points over the past year, and now play around 950-1000. About a year ago I hit my all time high of 1376, and figured I was just going to keep going up. I would at least always hang around 1250 at the lowest but never struggled to hold that range. Now I'm struggling to break 1000.
What happened to me? Is this a common thing? It's really been bothering me lately as I try and understand what changed in my brain. On the other hand my puzzles are higher than ever as I recently past 2000.
I've read similar threads before, one that stands out to me where a player played around the 2000 level, took a break, and then was only ever able to play around the 1100s.
Has this happened to anyone else out there? If so, what are some ideas on what could cause such a thing? They say practice makes perfect, but with Chess I've realized that is not the case, at least with me it seems. I'm just really confused. Thanks for any insight.