Sorry to hear about your difficulties. Maybe you can describe your experience in more complete detail, but the only way i can imagine you passing out is that even though you are getting up slowly and cautiously, it is not slow and cautious enough. I have sometimes taken 3 minutes to safely stand after sitting, surveying carefully each tiny step of the way how light headed i am beginning to feel and even once i am up and walking i am sometimes leaning against the wall and resting along the way or sitting back down if i begin to feel too light headed. For some people in some instances i would speculate that even 3 minutes of caution would not be enough. Getting some deep breaths before attempting to rise is another helpful factor.