Hi L and welcome:

First, the TDEE calculator at the top of the page puts your TDEE around 1990 cal. per day. If you have been eating to 2400, that will explain most, if not all, of your failure to lose weight. Electronic trackers are well known (proven) to be very inaccurate. I would never believe one, or base my eating on what they say.

Second, you can take heart that there are no known ‘detrimental’ effects to ‘ruining your metabolism’ like illness or death. The only apparent result of a ruined metabolism in the dietary world is a lower TDEE. That means your body has become more efficient and does the same work as it has done for decades with less energy (fewer calories). Many would applaud a result that caused the same work to be done with less energy – the impact on the world would be very beneficial. In the weight loss context, however, it seems that becoming more efficient means the person cannot eat like they used to eat without gaining weight. Of course, when they were eating like they used to eat they were gaining weight, so it is unclear why they would want to go back to eating the way they were – at least if they wanted to maintain their weight loss.

Anyway, the basic cause of a long plateau where you are neither gaining nor losing weight is consistently eating to your TDEE. That, of course, is the definition of TDEE – the amount of calories you need to eat to neither gain nor lose weight. So if you want to lose some more weight, and regardless of what your electronic tracker (or the calculator at the top of the page) says, you need to eat fewer calories than you have been eating.

Here is an explanation of TDEE: https://thefastdiet.co.uk/forums/topic/tdee-for-the-curious-or-why-dont-i-lose-weight-faster/. And here are some thoughts on plateaus: https://thefastdiet.co.uk/forums/topic/on-plateaus/

Good Luck!