As a new user of this site, I was highly motivated to play and entered many tournaments. I managed to get my rating up to 1790 and was hoping to go higher. But then my enthusiasm waned and I found myself not able to win so many games and I found it harder and harder to carry out the necessary analysis prior to making a move, so I made quick moves instead and slumped to 1720.

I wondered if this was a common phenomenon and, if so, how do people get themselves remotivated and enthusiastic and thorough again?

The less you worry about ratings the better you will play, honestly. In OTB games I never look at my opponents rating before the game and seem to play way better.

As for lack of enthusiasm, just do what is fun for you, if nothing in chess interests you take a break for a while and come back when its fun.

Edit: Frogot to mention that chess "burn out" can happen if you have to many online games on the go and it starts to feel like a chore.

Thanks for that. You are right, I did have too many games on the go and I think that is what triggered my sudden lack of interest. I am going to reduce the number of games I have from now on. I will also take a break in between tournaments.