The first thing that jumps out when reading your post is that you can stretch more when you’re relaxed.
The human penis has a couple of different layers that can potentially influence the elasticity of the overall unit, either individually or [more likely] all working together. There’s the dartos fascia, which is a smooth muscle layer just under the skin f the penis and scrotum which shrinks everything down when stimulated by things like cold temperature, fear/anxiety, stimulation of the rectum [like when you have to poop], or sexual arousal. It’s function is much more appreciable in the scrotum than the penis, but it’s not entirely negligible.
Then there are the deep fascial layers, the Buck’s Fascia and the Tunica Albuginea. Both are types of deep fascia which are attached to small skeletal muscles at the base of the penis, the bulbospongiosus, ischiocavernosus, and perineal muscles. When those muscles contract, they actually pull more of the penis inside the body. But one other important feature of deep fascia, which is only recently being discovered and isn’t well understood, is that while the tissue is mainly connective tissue matrix which is created and maintained by fibroblasts, the fascia also contains myofibroblasts. Myofibroblasts are cells that are almost like hybrids of fibrobasts and smooth muscle cells.
They’ve been able to demonstrate that myofibroblasts contract and relax in response to sympathomimetics and nitric oxide, respectively. Myofibroblasts have also been implicted in deep fascial ability to adapt to stress, becoming thicker and tougher when needed.
So, yeah. That’s a lot of biological jargon… What it means is that the layers of tissue in the penis that comprise the sheaths holding everything together have the capacity to be relaxed and pliable and to be tense and resistant to mechanical stress; and our body tissues don’t do a very good job differentiating between physiologic stress, mechanical stress, and emotional stress.
My guess is that your Buck’s Fascia and/or tunica have a greater concentration of myofibroblasts, are more easily stimulated to be contracted and “tough”, or both.
As for how you might overcome all that for PEing purposes is the real question though. My first line “treatment” would be to try to induce relaxation with both heat and mental relaxation. Failing that, one could try meditation to relax or even pharmacologic intervention. You’ll also want to maintain a healthy diet and exercise to maximize your endogenous nitric oxide levels, promoting both vascular as well as avascular relaxation.