Hey brother, first of all, congrats on 6 years clean. This is a big achievement. Possibly the biggest of your life and as long as you put your recovery first, you can take your bodybuilding as far as you like. One day at a time.
As for your concern, I know exactly what you are taking about. I do not believe you are injecting directly into a vein though. I aspirate every shot and after years and years, I have not hit a vein in the glute. However, I have gotten an instant "taste" of gear, as I am injecting. These thin slovents can be rapidly absorbed into the body, espeically with the newer, more cutting edge solvents that you find in thin products like Geneza. I believe that if it is injected close to a source of blood transportation, solvents can easily get picked up. Take your time pushing the oil in. If you begin to feel like you're close to a blood vessel, stop your shot, pull out and start again.
Still, lets work on your technique because I'd feel much better if you were confident with your injections. If you can't comfortablly reach back, try lying on the bed, where you can be stable and bring your leg around, better exposing your outer, upper glute. Then once you plant the pin, it should stay there without you holding it, allowing you to reposition your hand to aspirate. I'd also suggest that you begin using more injection sites. Over time, even if you only inject into a site 1x wk, you can build up scar tissue. So look into the outer upper quad as well as delts. These are easy shots. You want to plant your shot deep into the muscle. I'm not sure how you could miss the glute, but even if you missed it totally, the gear will still work. In recent years, some HRT Drs have been experimenting with sub Q shots. Still, I would prefer that you burried the pin, in the intended muscle. Injecting shallow into the muscle can cause more swelling and discomfort than usual