I have somewhat similar hair (fine/straight and also wear it fairly short - although I'm growing it out some) and it also shows every "mistake." I have a couple of suggestions:
Buy one of these:
http://www.amazon.com/gp/product/B001DYM62C?psc=1&redirect=true&ref_=oh_aui_detailpage_o07_s00One side cuts more, one side cuts less. Start with the less side and just run it over your hair. It's a good way to cut a little at a time without any specific impact and build up your confidence. If you take too much off, you can use this to make that transition area look less severe.
Add a lot of texture to your hair (think choppy pixie cut). If you cut lots of texture in, when you "make a mistake" it looks more purposeful.
Do a search on youtube for specific hair parts. Example - "cutting highly textured bangs" or "cutting the back of your hair." Watch a video and then click on similar suggestions on the side panel. Even if it's not a perfect video for your situation, you will learn more from watching it.
Start with a little bit at a time and then cut a little more. Don't try to go straight to the length you think you want. Ex. on my bangs, I cut in the texture first and then start to shorten up the length a bit at a time to get to the final length I want. Then if it's a little crooked, it doesn't matter because you still have some space to work with.
Keep checking out the cut and modify it as you see stuff that you want to change. The nice thing about cutting your own hair is that you can change it a bit every day if you want. You don't have to wait until your appt next month.
Finally, remember it's hair and will keep growing. So if you mess up, it will grow out again pretty quickly. It's not permanent!
Good luck!