This is a great question and one not many people will want to ask themselves. So many players just love to put it down to running bad and I have often said "maybe you are not on a downswing and just a losing player"

So an obvious thing to check is your EV. If you are playing cash just pull up the EV line and see how it looks. Now the issue with this is it does not nearly tell us the full story as a player. This comes down to the sample may not being big enough or just so many other factors.

Which games is it you play? How big is the downswing?

I think tbh one way to know is to either just if you know yourself you just ain't playing well and also just having someone review your game such as a coach.

I play NL10 at MPN. The swing is almost 7BI, but the sample is really small. I guess it's pretty clear that I'm playing bad (looking at EV line) and I think that the major problem for me is admiting it (I study poker more than I play it).

First you must understand what is Variance. For that you need to know

Winrate in BB / 100
Standard deviation BB / 100
Number of hands

Its all you need for simple variance. If you are good player your BB/100 in long run must be +. If you have 1bb/100 you hit higer variance than 10bb/100 player. Standard deviation is about which game format you play. 6max has higher standart deviation than Fullring. And lastly 1k hands higher variance than 100k hands.

If you are normal winning player in NL10 with 3BB/100 in 10k hands you still has chance to lose your bankroll 38.2089%.
NL10 with 3BB/100 in 100k hands you still has chance to lose your bankroll 17.1391%

You are in first line you has only 10k hands and if you are winning player you still has very much chance to lose your bankroll.

It's not often someone asks if they're on a downswing while posting a graph that ends in the positive

Over this sample it's nearly impossible to tell and 7 buyins, while it might seem like a lot isn't that much if you play cash regularly. As much as I'd like to avoid them, I have tons of 10+ and 20+ buyin downturns.

If you're not sure about some of your plays I recommend you post them in the HH review forum here. If you're making big mistakes, we'll find them.

A big thanks to both of you guys. Actually, I KNEW both things you said me, but I had to be REMINDED of them. It's pretty hard to deal with variance. It's pretty hard to deal with emotions. I am trying meditating to cope with that? And how do you deal with that?

For me, this is probably the hardest part of the game. If you do not control your emotions, you can lose the profits of couple of weeks solid grinding in couple of hours. I quit immediately after losing 3 buy-ins. This is probably too conservative for most players, but I do not handle my emotions very well in poker tables, so I just have to be very strict on this rule. This is kinda funny, because in general I am a very calm person, but in the poker tables it is another story. I am a member of a coaching site now so I feel like I always have eyes behind my back watching me. It actually helps, because now I feel ashamed if I spew any chips. This is why my stop loss limit is so conservative. I know I can lose 2 buy-ins without a big effect on my game, but the third is just too much.

These are exactly my feelings. I can call it the hardest part of my game too. I am so concerned about winning and losing... If I lose, let's say 0,5€ I feel kinda bad and if I win 0,5€ I feel like I played well, despite that I know, that monetary things doesn't reflect how well you play in the short term.