
What is the purpose of doubt & self-criticism?
It exists, which means it came from somewhere, which means it evolved to serve some sort of purpose...so, what is it?
There are a few things I'm going to hit on here:
Relationship with yourself
Grounding in reality
Overcoming thought distortions
1. Relationship with Yourself
First off, if you don't know already, you are always in a relationship with yourself. can prove it.
You can think about and describe yourself, right? You can judge who you are and what you do. This ability to self-reflect, to be self-aware, to talk about yourself like you would anyone else, proves that there is some third-party observer within us.
Let that sink in.
You are constantly watching your every thought, feeling, and action. This would be like spending every waking moment following someone's instagram and evaluating what you think and feel about where they're doing...sounds exhausting, right? Thankfully, it's automatic. It doesn't take any extra effort because its our own experience.
Now that we've established that...what is your relationship with yourself like? Are you kind, loving, forgiving, understanding, compassionate, encouraging, and accepting? Are you hurtful, invalidating, cold, judgmental, discouraging, and punishing?

Are you your Best Friend or Your Own Worst Enemy?
2. Grounding in Reality
Next up, whether you think you're God's gift to earth, Satan's curse upon the land, or somewhere in between, are you grounded in reality?
Here's where other relationships come into play–and this is a bit of a mindfuck. Yes, we are in a relationship with ourselves; however, we cannot truly see beyond our own experiences. This creates a problem: a runaway feedback loop. Without an outside observer, there is no objectivity, and we need that, otherwise our reality is purely subjective.
I'd like to introduce you to some fun terms: BIRGing and CORFing.
BIRG = Basking In Reflective Glory
CORF = Cutting Off Reflective Failure
These are well-established psychological theories, dating back to 1976. The simplest and most popular examples of these are sports fans:
We won!
The team lost.
AAAANNDD the flip side (my own personal twist) BIRFing & CORGing (Basking In Reflective Failure & Cutting Off Reflective Glory, respectively.)
I know what you all are thinking at this point: OOF! MY SOUL! DON'T HIT SO DEEP, BRO!
The point here is that we tend to let our emotions get in the way of the objective facts...
"I failed, but it was someone else's fault."
"I succeeded, but someone else could have done it 100x better."
Coming back to the facts: I failed, I succeeded, drop the story, stick with the facts of the outcome, right? See where I'm going here? It's rarely ever just the facts, there's always a qualifier.
Oftentimes this is where relationships with people outside of ourselves are valuable to collect more objective information. If we over-state our success or under-value our achievements, if we under-state our failure or over-value our success–we want someone in the wings to give us a–say it with me now–R E A L I T Y C H E C K.
You are probably neither as great or as terrible as you believe you are.
3. Overcoming Thought Distortions
Which leads us to point #3: we think, we feel, we perceive, we judge, it's not real.

Here's another tricky part. Reality is what it is. Experience is a filter. As soon as objective reality is experienced, we filter it through our experience. Our experience is informed by every memory, relationship, conclusion, and more to create our beliefs.
Thus, Experience = Belief
One more time for the people in the back...
Experience is Belief
So, what are you to do? Apparently you simply cannot experience objective reality. That's right. Well...mostly. Ready for mindfuck #3? Awareness of awareness can cancel-out awareness. The more you are aware of what your filter is, your experience, your perception, your beliefs, the more you can be aware of how information from reality is being filtered.
Big Takeaway: whatever you think, whatever you believe...you're wrong...AND you're right?
But...wait...what?...How can I be both wrong...and right?
The simple answer: yes.
The complex answer:
The only world we can live in is the world we experience. There is no way to experience anything but what we experience. So, whatever you believe...that will create the reality you live in. At the same time, everyone else has their own beliefs and their own experience. Just because you or someone else believe differently, doesn't mean one of you gets deleted from existence for being wrong. They continue on. We all do.
Now, if you're smart you may connect the dots here and realize that all these different people have all these different beliefs and we're living in the world together so reality is nothing more than a collectively-agreeable subjective experience which is wholly separate from objective reality while still validated by the fact that any realization thereof does not make any one of us wake-up from The Matrix AND...
It's okay.
Good work. Take a deep breath. Drink some water. You still have to go to work tomorrow. Don't let the existential dread spin you into powerless apathy.

Choice.
You choose your reality.
What reality do you want to live in?
Your Doubt and Self-Critic serve a purpose. That purpose is to S.T.O.P. (I Ching #64, 2nd Changing Line, Peter Crisp)
Stop
Think
Organize
Proceed
When you are feeling doubt. When you are feeling critical of yourself. That is something within you telling you to take stock of your situation. Is this the right path? Are you taking the right steps? What are the dangers to look-out for? What can you learn from the past? How can you plan for the future?
This is the other part of yourself reaching out to have a conversation with you. Your self-critic will run rampant if you don't help ground it in reality. It will convince you of emotionally-distorted information if you're unaware.
KEY POINT: IT'S TRYING TO PROTECT YOU FROM EXTERNAL HARM
(albeit by using internal harm)

Your self-critic and doubt are not something to be trampled out of existence. Without them, we are un-grounded narcissists who will blindly rush into situations doomed for failure.