You're going through some real anxiety about pinning bro. If I may offer any advice... calm down. Relax yourself before you inject. The great and overwhelming chance is that you are going to be fine. Even if you hit a vein, it's not the end of the world.

By the way, if you DO hit a vein, you WILL KNOW. So don't go guessing "man I probably just fucked up...", just take your time, inject sure and steady. Aspiration is always good and in the big picture neccessary but the chances are you didn't hit anything unless you have a huge twitch and contraction from hitting a vein.

Expect a little pain from the "foreign liquid" going into your body, and from the needle, although a 25 guage isn't too intense.

Good luck!