I get worse as tournaments progress, why is that? (Real Life Tournaments)

Hello,

after finishing my last tournament in which I got 2/2 against really strong opponents and after that 1/5 against mediocre-strong opponents, I got suspicious as I've always had a feeling that I play better in the first rounds than I do at the end of a tournament.

Because of that, I analyzed all tournaments I've played in 2018. I wrote down the opponents and the results for each round and calculated my perfomance in National ELO (the data was just easier to access and there is no defining difference to International ELO)

My current National ELO is 1988 and I am 16 years old (I think this might be a factor)

So here are the results:

As you can see, there is a 500 ELO difference between my performance in the first round and my performance in the last round.

Do you guys have an idea, why that happens and how I can fix this? 

1. Maybe you have played too many games vs mediocre players and your level of play adjusted to theirs.
2. Playing better in the first few rounds of the tournament can be perhaps due to you losing focus and getting tired. Chess requires calculation and thinking and it's taxing on the brain.
3. Ratings are supposed to be CONSISTENT. Meaning, you can be 2200 strong at times but if you lose vs lower rated players, you're not yet at the level of a proper 2200 player. Thus, your rating decrease can be attributed to your inconsistency.

Just 3 wild stabs from my perspective, take it as you will.

Cheers

I'm not saying, I am a 2200 player, I am not at that level yet.  But I believe, I could get closer to 2200 if I wouldn't become so weak in the last rounds. I mean, 1781 is way below my skill and if I would at least play around my current rating in the last rounds,  I believe my rating would be higher. 

To point 1: I usually play against stronger opponents. This year I only played two tournaments with players <2000 and these where the U16 championship in my Bundesland and a tournament to prepare to the championships. Normally, the only tournaments with many players <2000 I play are youth tournaments.

To point 2: This might happen, but it also happens to my opponents in the tournament and they are often a lot older than me so they should suffer under it more than I do (I also do sports in my freetime, so my stamina should be fine)

To point 3: I am not 2200 at random times, I am always this strong at in the first rounds of a tournament.

Anyway, thank you for your answer!

How well do you sleep the night(s) before tournaments?

I know you play sports, but do you get plenty of endurance-type exercise so your body's endurance is good throughout the tournament?

How tired are you at the end?

What do you eat and drink during the tournaments?

Another possible cause is: since you do well early, you're playing others who do well early -assuming you're in Swiss-type tournaments- and that may put you against players with lower ratings who have improved or are having a good day and so you don't score as well against them as your rating difference would suggest.

I'm guessing fatigue causes a lack of focus. 

Do ya still struggle with the same thing, bc I do too!