In general, the needle is the size of a diabetic insulin syringe and nerves are not particularly sensitive in the penis, but then there's the whole psychological issue of poking the penis and also the comfort with injections in general, particularly self-administered. As a mental health professional, I can work at desensitization and persevere, but discomfort with the injection process remains the biggest reason people don't use trimix or stop using it. I am "happy of myself" (google youtube video of boy learning to ride bike) that I have persevered for over two years now.
To avoid scar tissue, it is important to vary the injection site. I do this systematically and have avoided scar tissue so far as I know. I use a clock grid (9,10,11,1,2,3) and a finger length from the base (two fingers or three fingers) and follow a chart....

It feels like just a tiny prick (sorry for the pun) and the injection takes a few seconds. The thought of it is far worse than the reality.