I tend to be very conservative especially when it comes to practicing Secret School Dharmas such as mantras. In my humble opinion, one should not pick up mantras on the internet to practice for many reasons. I’ll name just two here:
For a complete copy of the Great Compassion Mantra, please visit our web site. I’ve included the link here for your convenience: http://www.chanpureland.org/texts/. Select “Daily Recitation Handbook,” you will find the mantra on page 69. More importantly, you should find a competent sanghan to officially transmit it to you before you begin practicing it.
To practice, you can start with reciting the mantra 21 times a day and eventually increase it to 108 times a day.
My sympathies to you for your illness.
When people are sick, they seek out a competent doctor’s opinion. Similarly, you should personally make an appointment with a Good Knowing Adviser and ask for instruction on how to deal with your illness.
Then, you’ll be taught how to cultivate to obtain help from the Dharma. The more sincere you are, the more likely that you will receive better instruction and obtain more responses.
Until then, I suggest that at the end of each day’s recitation, you dedicate the merit and virtue from your cultivation to those beings that are tormenting you so that you can get better.
My best wishes!