It sounds like you are doing really well! Kudos to you!

It sounds like your progressing at a nice steady rate, which can be almost bittersweet in that while it is fairly rewarding and enjoyable to see results , it can still be aggrivating when you have to wrap your mind around newer, more challenging concepts.
It's actually funny that you mention the three chord progressions, because I still remember doing them to this day. I think the best way to progress is to focus on technique and getting a solid foundation. Make sure you feel comfortable as you attempt some of the things you are doing. (Hand positions, extention of hands/wrists etc.) It is important to make sure you are relaxed and all tension is vanquished. Make sure your hands are ready for your exercises. As Becky suggested, breaks are always a good thing. Multiple breaks will do wonders.(Even if it's to get a drink or do a quick task around the house.) Be sure to stretch your hands and arms out regularly, and make sure you don't ever play cold! Because if you strain something that would not be alot of fun...
As for the practice time, I always look at it this way- Make sure that you play enough to learn something new/beneficial to you personally, then leave off the piano just as you are really overcome with a joy for it. Then you will finding yourself really looking forward to going back on. And too, slowly you will find your periods of practice gradually start to lengthen as you see the fruits of your hard work!

But anyways, don't give up! I know, sometimes it can seem challenging

, but I think that your teacher pushes you because they are excited to see what you are capable of. Try to be a figurative sponge. Take in the things they say and any suggestions that they may make. If you feel you aren't quite ready for a new step or need to clarify something from your previous homework( technique or artistry) feel free to vocalize it to your teacher. Explain how you feel and ask lots of questions if you're curious. This shows the teacher that you are interested and it also ensures that you get the answers you need.
I think you are really doing great. I myself am a later starter as well. I'm hoping to become a teacher one day. So I view every single day as a stepping stone in my musical journey.
But never stop. Do what you love. Love what you do.
Hope this helped.

All the best.