A few other things to try ...
There is a book called "Too Good To Leave, Too Bad To Stay" that has an approach that I have used in many situations, the PhD dilema being one.  On one column put "I would continue the PhD / Masters Program because ..." on the other column put "I would quit the PhD / Masters Program because ..."   Then just quickly put down your thoughts and feelings in the appropriate column without trying to analyse or edit them.  Having done this, see if you can write down any thoughts or feelings that might counter what you have just wrote.  For example, one of my entries was under the Quit side - I feel deprived of being able to be involved in "fun" activities like working on my model airplanes because I have to work on the thesis.   Later, I added on the other side "I am prepared to sacrifice in the short term for the benefit of the long term"     Anyway, you get the picture.   It helped me to flesh out my deeper feelings, and the completed form also helped me to talk over the issues with my wife.

I also have collected a few quotes that have helped guide my thoughts. See if they resonate for you.

We can’t solve problems by using the same kind of thinking we used when we created them.
Albert Einstein

He can who thinks he can, and he can't who thinks he can't. This is an inexorable, indisputable law.
Pablo Picasso

I am always doing that which I cannot do, in order that I may learn how to do it.
Pablo Picasso

Ciao
Bulldog

It must be karma that I found this site.  I am trying to complete my research proposal and prepare for the candidacy exam and some of the key words in the messages jumped out at me. Not qualified, lack of self confidence, not ready  - to name a few.   I'll add "Do I really know what I am doing?"
Last weekend was the meltdown. Now I think/hope I am back on track.  Key message to self and others - "Baby steps"   Also, there are "Talks" on line by Jeru Kabbal that might provide insight into self. Realizing our own, persistent 4 year old and the chatter that goes on with that.  How much external validation does it need?
Not that I am a guru of self realization or anything, but I am 2 years into the PhD, turning 64 years old on Monday and sometimes I still can't seem to get it all together.
Situation normal - get used to it - trust in yourself. If you can, try to adopt the motto "I will do whatever it takes"
Bulldog