Hello all

I'm new to this forum, though not new to meditation. I've been practicing different methods for nine years or so, though I am still struggling with the issue of posture while sitting, and was wondering if perhaps someone here could give me advice on how to improve it.
I've tried several different postures over the years, all of which give me discomfort to the point that I'm not able to relax and get comfortable, and often just suffer, tense, through much of the sitting period. Recently I've been sitting on a chair with a little pillow under the edge of my butt which helps to keep my back straight, though this is not optimal since it's not very stable, and is uncomfortable (though not as uncomfortable as "tailor posture" has been to my back, or kneeling posture to my knees).
I know that dealing with pain equanimously is a huge part of the practice, but at the same time the point isn't to torture yourself. I think it will benefit my practice tremendously to be able to sit in a posture that is comfortable, stable, and allows me to keep my back erect.
I'm aiming for the burmese posture, because I know this allows the stability of sitting on the floor, while keeping the back erect, and is the easiest of the recommended floor postures (the lotuses can wait for now

first let me get comfortable with anything!).
So can anyone recommend some kind of practice, stretching, progressive plan, etc., I can do which will train my body to sit in burmese comfortably?
Many thanks!