I guess that I had better preface this by saying that I am not a PT or similar health professional and am only relating my own experiences.
You may like me have had a high hamstring tear/strain.
I was not helped by rest, ice, stretching, anti inflammatories,ultra sound or electrical stimulation although any combination may work in your case.
As I said deep cross friction massage did work on a hamstring injury which had become chronic over a number of years. So yes it can feel as if it is an insoluble problem but yet a cure may still be obtained.
If you have had the injury for 9 months I doubt whether rolling on a ball will cause further damage. Cross friction massage has become fairly mainstream for treating hamstring injuries and excessive caution is probably misguided. My own PT treats international athletes and the approach has proved successful.
I was in so much pain when my therapist massaged me I couldn’t say for sure what he did but as it is meant to be “cross friction” I guess that gives a clue as to the direction of the pressure although there was a lot of direct pressure as well.
When I did it for myself I used a TPT ( trigger point therapy ball) rather than a tennis ball. Similar size but holds it’s shape better so gives more direct pressure. I do not know enough to say that there is a substantial difference to warrant the expense but simply bring this to your attention.
In any event I sat and rolled around on the ball trying to locate tender spots and when found easing my weight down on a specific point as much as I could. In the beginning I would often need to support myself to take the weight off as it was so tender and painful but over time I could put more weight on and then fully sit. I think the TPT protocol is to locate and then apply static pressure for a minute or so on any tender point. Generally I found that 5 minutes or so massage was enough but I did it every day. I did not use ice.
You should be able to trust yourself to locate where you need to apply pressure and for how long.I had a lot of pain around the
bone where the hamstring attaches and this may be a tender area for you as well. You can roll up and down, across and around the point and then apply static pressure
I cannot stress too much that this type of massage is or should be very painful and bring tears to your eyes. If you find it relaxing you are not getting the right thing.
One final thing. My therapist had me run progressive sprints after 2 massage sessions to complete the rehabilitaion and I think that this is something that Charlie Francis prescribes for hamstring injuries.I did not follow this protocol when I self treated.
I hope this avenue is fruitful for you. I know how frustrating persistent injuries can be. See an athletic trainer experienced in giving cross friction massage if at all possible but if not see whether you can get any relief as above.