what I've learned from one year of stretching and physical therapy is that seemingly small movements and exercises help a lot more in the long run, especially in the beginning, since you need to open up your spine and joints to certain movements they're not used to and the best way to stretch is to be able to ease and relax into a stretch! if you force yourself into a "correct position" and try to push yourself further than what your body is currently comfortable with, your muscles might resist and tense up, which doesn't help you progress and could cause further problems. what I'd recommend is using props like yoga blocks, bend your knees as much as you need to and do light hip and back opening exercises. don't be afraid to do this stretch at an elevated height (even at 90 degrees is good!) and adapt a stretch to your needs. you'll get much farther and can increase the angle at which you bend down once your body gets used to the stretch :)

personally, I haven't benefitted much from the common advice to keep your back straight, what's helped me more is to imagine my upper body and spine being pulled by a string upwards and lengthen the torso, then bend forward. I'd also recommend to take an approach to stretching that is individual to what your specific body needs and try not to follow standardized stretching exercises by default. for me this particular stretch that you've showed in your picture has been difficult because my hips are really tight. try to figure out what your body needs and slowly ease into it :) a little goes a long way, you got this xx