Is there a stronger word for "highly frustrated" or "feeling very hopeless"?
So If I want to tell someone that I am highly frustrated by the work I am doing or convey that it's very demotivating, what are the alternative words I can use there?
You could try “despondent”:
Adjective: despondent (comparative more despondent, superlative most despondent)
In low spirits from loss of hope or courage.
I would suggest "exasperated" as a synonym for very frustrated and maybe "soul-sucking" to describe work that is very demotivating.
References: List of synonyms for 'very frustrating': https://www.thesaurus.com/browse/most%20frustrating
For 'soul-sucking': (the link is to someone's blog, which illustrates the phrase being used in context) https://btr.michaelkwan.com/2017/12/01/soul-sucking-habits/
Perhaps you are "at wit's end":
The limit of one's sanity or mental capacity; the point of desperation.