Why that last 10 pounds is so hard to lose

The ultimate answer to why you plateau, why that last 10 pounds is so hard to lose, and why it's hard to breakinto those single digit body fat levels is that you were in a calorie deficit before, but for all of the reasons previously mentioned, you're no longer in a deficit.
The way to break the plateau then is to:
1). Re-establish the deficit.
2). Re-stimulate metabolism and re-set fat-burning and starvation hormones (if necessary)
3). Keep after it! (persistence, persistence, persistence).

Before you think about what strategies you'll use to re-establish the calorie deficit, there's one thing you might want to do first. If you've been dieting a long time, give your body a short break from all the calorie restriction. Raise your calories to maintenance level (meaning: the amount of calories you need to maintain YOUR CURRENT WEIGHT) for at least one week and up as long as two weeks. During this break, you're obviously not going to lose fat, and you might even gain a little scale weight from water and glycogen. You have to be okay with that and think of it as one step back for two steps forward.

A week or two at maintenance calories has mental and physical benefits. It gives your body a physiological break from the challenges of maintaining the deficit. The break also resets the "starvation hormones" and stimulates your metabolism. Then, when you go back to the calorie deficit, your body starts responding again like it's supposed to and you can make that final push to a six-pack abs level of leanness.

Because so many people underestimate how much they eat, if you hit a plateau, you should start tracking your food intake. Note your meal plan on paper, with the calories and macronutrients subtotaled for each meal and totaled for the day. Then weigh or measure your food portions to be sure you are hitting your daily calorie and macro goals.

You may have been told many times by a lot of different experts that you don't have to count calories to lose weight and you may even know people who lost weight simply by cuttiing portion sizes. But if you're stuck at a plateau, I consider it mandatory to stop guessing and start getting serious about doing your nutrition by the numbers.

The Best Ways To Re-establish Your Calorie Deficit: Once you're tracking everything you're eating so you're sure about your compliance to correct calorie intake, you need to re-establish your calorie deficit.In other words, you need to eat less, burn more, or a little bit of both.
You can work on the nutrition side (calories in) by reducing your food portion sizes and choosing less calorie-dense food types. Or you can use the exercise side (calories out) by increasing training volume. You have many choices on the training side.
If your weight training has been low in volume or frequency, you can do more weight training. A 3 day a week program can be expanded to 4 or 5, increase sets and exercises, more metabolically-demanding exercises can be used and intensification techniques can be added.

You can increase your cardio duration (burn more calories by working out longer) or increase your cardio frequency (burn more calories by working out more often). You can also increase the intensity of your cardio so you burn more calories in the same time you're already spending (a good strategy for busy people). That's three different strategies for cardio alone (duration, frequency and intensity).

The whole idea is simply to expend more calories (and stimulate your metabolism) and pull back your calorie intake, which re-establishes or increases your calorie deficit.

You could make one change at a time, or make multiple changes all at once. Which strategies you choose depends on what makes the most sense given your current situation. Always remember that there are two sides to the energy balance equation (what you’re taking in & your expenditure), not one. There's also more than one way to break a fat loss plateau.