Sitting on thr floor upright is painful for my back. Are there any suggestions on how to improve my meditation posture?

I usually sit against a wall or side of my bed when meditating. My back feels comfortable this way so that it doesn’t distract me while meditating. I also try to slightly engage my core when sitting so that I can keep my body alignment straight.

I had the same issue! You can try to sit next to the wall or sofa's back side which will help you to support your back. Also, try to increase the duration gradually so that you train your back muscles gradually. It also might indicate that your standing posture is incorrect and you might need to focus on it as well

If your current posture causes pain, the way I see it is that you have 2 clear options; choose another posture (as in sit on a chair, or lay down) or focus on the pain compassionately. Allow it to be there without resistance. Don't fight it, explore the sensation. Move a little to see if that helps….
You've got this!