when do people start a feeling "May be, I am not fit for coding".

Sometimes, it's shortly after this moment, that another moment happens.

We call the other moment the breakthrough moment. If you are persistent enough to experience it, you'll forever refer to it as the day you became a programmer and either started hating or loving your life.

Hang in there until that other moment, it's usually worth it. Good luck.

Every day. Then I fix a pesky issue and I'm happy again

Everyone in every field will have that once or twice. "Do I want to do stuff like this for thirty, forty more years?" it's a slur you'll have to work through.

When everyone tells you that you're shit, and you start believing it. When you sit down and struggle, and everyone else is like, "haha wow yeah hey, that wasn't hard at all."
When you're financially fucked, socially fucked, and can't think straight because you're stressed, all because you chose your dream above a choice of doing something you hate. When you're human, and are different. It's a good thing.

Only when you feel like this, is when you can truly look at yourself and then decide between those two decisions. "fuck it" or "fucking bring it". You can do anything you put your mind to, so why not do something you find difficult but rewarding.