If you got extremely lightheaded and your legs began to spasm...are you sure you didn't pass out? Time wouldn't appear to pass while unconscious, so you may have passed out at 20, woke up, and counted 21 like no time had passed.
But let's suppose you didn't pass out. Then you didn't apply enough pressure around your carotid arteries consistently enough. That could mean that you didn't lean forward / down into the rope enough (could be a posture issue, could be a sitting-in-a-chair issue - you want your body weight to be doing a lot of the work); it could mean that your bathrobe belt is too wide, or too elastic, and a thinner rope might be better; it could mean that the belt was too low down your neck (did you feel like your head was going to explode without also feeling extremely lightheaded? Pulling the belt up higher might help).
To expand on the posture/sitting-in-a-chair issue - for reference this image explains how much of your body weight puts strain on the rope by body position. If you were sitting, you were probably closest to position C, where it could be that only 17% of your body weight is bearing down on the rope. If you got into a kneeling position, you'd apply much more force around your neck. Note, though, that it's dangerous to test!! Please don't test this unless you're sure you want to ctb.
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I know you're in a lot of pain, and I hope that whatever path you choose, you get to have a little less suffering than you have right now <3