I’ve found the longer I stay awake the more flexible I am. Add in regular stretching and most regular sized people can do this. I am too fat now but I was able to suck myself to orgasm pretty often. You can either sit with your erection up with legs tight together flat straight ahead and use your arms under your legs to pull yourself to your cock or you can contort yourself by laying on your back and lifting your legs over your head and using the same arm technique. I’ve also done it similarly in the doggystyle position. You got this!