I found that an 80% adjustment to the SRL was good for me.
Also, clinical research has shown that most men sleep in a fetal position. I've been able to do that in the Grand Trunk Ultralight, though it is rather narrow and my new sleeping bag allows it though my old one did not (which got converted recently to an UQ).
Usually we change positions in sleep several times during the night, often not even knowing it. But if we sleep with fear of falling out, which I did the first couple of times in a hammock, then the stress alone might cause back and shoulder pain, or a headache ... or however your body registers stress.
But hardly anyone falls out of bed, right?
A hammock seems to be the same. So I have learned to sleep with confidence, if that makes sense.
I have a theory that there is a subconscious sleep adaptation that can eventually accept a new "bed." Soldiers on campaign, for instance, can learn to adapt, even when noise, chaos, uncertainty, or threats to life and limb surround them.
Research has shown that it takes 48 hours to acclimate to a new or changing situation. I intentionally spent three straight nights in my back yard in the hammock to be sure it was for me. In he process I discovered a better pillow for me, even, so it has been a great and enlighteneing success. Human psychology is deeply connected to the need for sleep (Carl Jung). So, keep trying.
I expect that hammock-phobia would disrupt life goals at about the same rate as other disorders, less than 2%. That is about the ratio of those on this site that have expressed similar concerns as yours. It seems that one guy dropped out entirely. But as they say, admitting the problem is half the solution.
What is the motivation?
I found hammock camping because of injuries that require me to go ultralight. But some may be perfectly happy to stay on the ground and lug heavier loads. There is nothing wrong with that. The point for me is the love of nature and of nature's God. Whatever a man needs to do to get out there ... well, we are all brothers and sisters that love it.