You say you were overweight for about 3 years (when you were 100kg). Have you ever had a weight problem prior to this time? Yo yo dieting (start-stopping new ones all the time) can contribute to slow losses even when you're doing all the right things. Your metabolism gets more stubborn as you get older, although I'm guess the 81 following your nick is indicative of your age? In which case you're not that old.
I can feel your frustration. I managed to lose over 60kg in under a year (many years) ago, now it takes me a year to lose a third of that!!!!
Our bodies are creatures of habit, and sometimes changing the routine upsets our body more than our mind wants to acknowledge. If you lost 4kg in 3 months with just good eating, but then in the next 3 months only lost 1kg AFTER adding exercise, my advice would be to ease of the exercise, or at least, change the type of activity. You may be building muscle, which weighs more than fat, and although it may show in your measurements won't register on the scales.
I believe it is a matter of frequency rather than duration. That is, better to do 20 solid minutes 4 or 5 times a week than it is to do 1-2 hours 3 times a week.
But then, what would I know....I am over 100kg and trying to get down to double digits is taking foooooorrrrrrrrreeeeevvvvvver!!!
Keep at it though, and remember "a loss is a loss" no matter how small, and it is better than no loss at all !!
Strong people don't walk away from the truth, only the excuses others try to sell them.