Q:

〜ていない vs 〜ないでいる

I read many times this question but I'm still confused about the differences between these two grammars. The first answer says that 〜ていない has a possible third meaning not used, "I eat my meal and I am here (/I exist)" that is "a terrible way to read it", and when it goes to explain 〜ないでいる, it means "I am here (/I exist) without eating my meal." which seems very similar to the terrible way of reading it. 
Also, wouldn't ていない mean "... and I'm not here"?
I appreciate the efforts of the person who answered, but I really didn't understand it, especially what "instrumental adjunct" is. 
I'm not sure, but the difference between them seems to be how long the state remains. 
In 食べていない I don't eat something since a long time ago, it remains until now and it will remain in the future, 食べないでいる on the other hand, would mean that I'm in the state of not eating now, but that won't last too long.
The sentence that made me search about this grammar is: 僕が…書けないでいるから？  It's an answer from person A to person B when asked why the person A is in the same place as B.
Could someone explain the differences between these two grammars and when should I use it?

A:

Without complete predicate, it is hard to generalize the difference between 〜ないでいる and 〜ていない. So, I'd like to use 食べていない and 食べないでいる from your example.
And, I have not understood what "instrumental adjunct" states clearly,but I'd like to answer. 
The author in the link might be explaining the sentence : 食事を食べないでいる implies "I want to be in the state of not eating ( in order to lose the weight, etc. )" by stating "I am here (/I exist) by not eating my meal." (instrumental adjunct) 
I mean "I am here by not eating my meal" implies to complete some purposes of its action. So, 食事を食べないでいる implicitly modifies some purpose such as losing the weight, etc.
Consider the sentence 膝{ひざ}を怪我{けが}しているので、走{はし}らないでいる。 "Due to the knee injury, I try not to run ( the purpose : in order to get recovery in the knee, etc. )."
So, your sentence : 僕が…書けないでいるから？ is possibly saying "I am here probably because I could not write ( Due to the slump, person A does not have an idea how to finish writing. )".
食べていない implies basically "not eating at this moment". It might imply habitual action, progressive action, experiences, etc. such as "I do not eat X recently", "I am not touching the food to start eating", "I have not eaten since this morning.", etc.
So, 膝{ひざ}を怪我{けが}しているので、走{はし}っていない。 could imply "I do not run regularly due to the knee injury".

