Speaking to BBC News NI, Mr Donaldson said he joined Fraser Partners – previously known as Fraser Estates – in 1986 as a civil engineer, before being asked to take on a housing development management role in 2016.
After suffering two strokes, the 63-year-old, who lives in Belfast, took eight weeks off work before returning on a phased basis.
However, he said he “never in a million years” expected to be told his employment was coming to an end during a regular weekly meeting.
“It was such a shock to me that after 40 years of association with the company that I was let go,” he said.
Mr Donaldson said he was extremely disappointed, and his mental health suffered “tremendously”.
“Two weeks after I found out I was let go was the worst of my life because I didn’t know if I had a future at all,” he added.
“When you have a stroke, it changes your life, you wonder: ‘How am I going to look after my family?’
“You have no income, you don’t have a job. You wonder where the next step is coming from.”