I was speaking with a friend the other day. He's an artist who is going through some rough period of self-doubt and questionment. 
It's something every artist out there face at some point, that big, scary and sinister question : "Am I fit to be an artist? is it really my path ? my art seem to be so lacking compared to that other one" etc  It's very easy to get obsessed by those questions, to torture yourself and ruin your motivation over them.

I can tell every single ones of my artist friends faced it themselves and some continue to from time to time, even after they became professionals.  
Doubt in art ( and probably any creative field ) is completely normal. It's part of the job one could say.
And imho a large part of managing to become successful as artist is to bring down these doubts to an acceptable level ( you will never completely get rid of them ) and find confidence in yourself and your art

In my case it happened like this. Years ago I was very un-secure about my own art. I kept telling myself again and again I was not up for it and I came close a couple times of completely quitting.
Then one day, after many unsuccessful attempts and rejections I got my first illustration job ! A big thing ( that actually wasn't that big at all ^^ ) that I didn't expect and that overwhelmed me and magnified my doubts even more.
nonetheless I focused seriously on it and, somehow, managed to do in time and my customer told me afterwards they were satisfied. And after that everything changed !

just managing to finish it sort of unlocked something in me. I no longer was uncertain. I knew from now on I could do it. And after that my whole outlook on art and drawing started to become a lot more sereine. When you stop posing yourself those questions all the time, then you can focus purely on your art and you start to improve by leaps and bounds.
I started to be able to take on bigger works , draw progressively larger and more and more complex pictures. The questions in my mind were no longer "am I fit to even be an artist and a pro? "  and more practical and specific like " ok I need to do that in that angle. but the perspective is tricky, how am I going to pull that?"


unrelated picture but yokohama kaidashi kiko is awesome so there.

Here is a little advice I gave to my friend and that could work for you if you're an aspiring artist ( it has really worked for me with comics )  :3
Scale your work.
Set yourself objectives. You start with small ones in small projects. You achieve them and then you set the bar a little higher for your next one. What it's going to do is increase your confidence in yourself.  The important point is to manage to achieve the objective in question. You're not looking to produce the biggest masterpiece ever yet, you just want to achieve and finish it and reach the bar you have set for yourself. Once you do, you raise the bar higher, slowly. If you mess up, no problem !  you just decrease the bar a bit and you go again.
In my own case with comics the goal was actually to manage to finish them at all. at first I did two pages comics, then 5, then 10, then 20 etc
what it did for me is once one tier was passed, I knew I could do it and no longer worried about it. Without that process I simply would never have been able to draw large 100+ pages mangas for instance because it's such a scary and overwhelming prospect if you haven't prepared yourself for it. its a very psychological thing.

So yeah, the important thing to keep in mind is doubts for an artist are like art blocks ( made a journal about them here : What are art blocksI was speaking with a friend a little while ago about the dreaded art block.
It happens sometimes : you feel demoralized about your work, you feel that no matter what you try you are stagnating, you have lost your momentum and it turn your motivation off.
It's a pretty bad feeling but there is an important thing to know about art blocks :  They are a  totally natural process !
I don't know a single artist that don't have them from time to time. There are just days where all you do feel like crap and just the idea of drawing is painful.
I had my share of them so here is my personal take on them and how to fight them
The first thing that is important to mention is I don't think it's actually possible to stagnate when you draw regularly, let alone regressing.  Each time you draw, you become a little bit better , it's really as simple as that ! It varies from the type of drawing you do of course but there is always something you pick while drawing : you become faster a )
They're completely normal and part of the process ! :3
If you're an artist, sooner or later you're going to run into them. If you manage to bring them down and find your confidence, then there is very little things that can prevent you from going very far and realizing your dreams ;3