I've had a 32" waist forever and always maintained a good level of activity to keep it that way. A few years ago I had a course of mirtazapine which, I didn't realise at the time, made me put on quite a bit of weight so I was just about squeezing into 34 trousers. Ever since I've been trying really hard to get rid of the extra weight and I've dropped a bit but it just seems impossible. I exercise a lot anyway, go to the gym usually 5 times a week and cycle at weekends. I find diets really easy to stick to and have been through the whole range of no carbs, high protein, no meat, all meat and so on. For the last 2 weeks I've been on a protein shake for breakfast and dinner with porridge and fruit for lunch. But no change. I'm pretty sure I should be wasting away as I can't be on much more than a thousand calories a day. Can there be any underlying conditions that would make weight loss particularly difficult?
Partly age and slowing metabolism. Also the body trying to hold onto fat as it feels it is being starved by your meagre calories and high exercise. Eat some more with plenty of protein and 'good fats' like nuts and avocados.
"a protein shake for breakfast and dinner with porridge and fruit for lunch"
How few Calories is that ? The body tries to stabilise what it decides is your weight; you need to nudge it into change, not trigger a reaction to conserve energy.
I'd suggest joining a slimming club to get good guidance, good habits/system, and monitoring.
You also need to feed your body, and not worry about calories. What you are living on now amounts to starvation. Eat good, unprocessed foods, avoid white carbs, and eat to satiety. What TTT says about intermittent fasting is good, I also try to follow a 16:8 rule. Have a look at the Diet Doctor site...lots of good advice along with science about weight loss and nutrition. You may not want to go as low carb but it's still excellent information.
"Can there be any underlying conditions that would make weight loss particularly difficult?"
Yes there could. I know nothing about your past medical history, but i would have thought that an underactive thyroid should be excluded by a simple blood test.