I'm glad you posted @Frenchcremefraiche because aged 46 and not being able to get up off the ground or the floor at home is actually quite shocking. I'm also a bit shocked that you can't do 6 floors of stairs. I think you should do them all, not just 3 flights - can you allow extra time for stopping and catching your breath? (Alfred Wainwright great fell walker, wrote that a pause to catch your breath is always excusable, to enjoy the view!
Your heart fitness must be quite low as well. How far do you walk each day? Can you stop using a car, and walk for at least an hour a day (10,000 steps minimum?)
My PT trains me hard, and he also trains a woman who is heading for 90. We both do burpees (he made me do 67 on my birthday - one for each year). My gym friend (who's around 88 I think) does burpees very slowly, but the point is that she can get up off the floor.
So: burpees - do them slowly, break then down into the separate movements required (I wouldn't worry about the jump at the end). Squat, reach forward with your hands, full front body on the floor, then push on your arms & bring first one, then the other leg into a table top position (hands under shoulders, knees under hips, like a crawl position) and use your legs to get you upright.
The strength to get up off the ground is in your glutes, then your arms and back. My pT has me do all sorts of crawling movement patterns as well - good for mobility & getting you used to working your body at different levels.
You say you're doing strength training, but frankly if you're not strong enough to get up off the ground, it's not enough! Are you doing squats with dumbbells or a barbell? Or a weighted back pack if you're at home? Put books or tins of beans in it ....
A proper, full depth squat will help to mobilise your hips. Keep you chest & eye line up, keep both feet on the ground firmly, hinge a bit at the hips - push your butt backwards - then think about dropping your pelvis between your legs, keeping your chest as high as possible & your chin up& eyes up.
The child's squat is habitual in many countries (India, African countries) for many people who squat like that, instead of sitting on a chair. Google it, you'll see. Here's a good discussion of ways for older adults to squat (at 46 you're not actually "older" but your body is behaving as if you are):
https://movement-x.com/older-adults/three-variations-of-the-squat-exercise-for-older-adults/
https://www.hingehealth.com/gb/en/resources/articles/benefits-of-deep-squat/
https://www.google.com/search?sca_esv=5cf68b8f0ed15ed3&hl=EN&gl=UK&udm=2&sxsrf=ANbL-n52hwZY6yj8-Ls89Lla7sbG6YZQBg:1770040179377&q=squat+sit+pose&stick=H4sIAAAAAAAAAFvEyldcWJpYolCcWaJQkF-cCgA8GqfDEQAAAA&source=univ&sa=X&ved=2ahUKEwiw7YTB-bqSAxUaZ0EAHdGqBfcQrNwCegQIdhAA&biw=1413&bih=793&dpr=2&aic=0
This deep squat feels lovely for your hips. Once you're comfortable in it, you can move around a bit to stretch your sides, or open out your hips a bit. Or can you just sit on the floor more at home? I'm 67 and I tend to sit on the floor more than I sit on my sofa.
If you are doing yoga & pilates & can't get up off the ground, I'd be looking for better instructors, or I'd be working harder! You've had a wake up call, and it's not too late to do something to save your body.
Good luck. Movement is medicine, remember.