The 33-year-old was found dead in his Catterick Garrison room in January 2020 — three weeks after he was last seen.
An inquest heard L/Cpl Mongan told a colleague he felt “manipulated by seniors” and was being overworked in what he described as a hostile environment.
He had a history of depression and suicidal thoughts, and had recently been treated by military and civilian doctors.
A GP who saw him said he had reported drinking heavily, feeling low, and having fleeting thoughts of suicide, but later showed some signs of improvement.
Records show he missed a follow-up appointment in late 2019, and no further medication was prescribed.
The inquest, at Northallerton Coroners Court, continues.
