You CAN do it. When I went for my extra, I got the book, decided I needed some work, took some on-line tests, was averaging 'bout 70%... Yup, thought I could do it with some study. Checked for when the next test was scheduled... Whatdoyoumeantomorrow? Crap.
Decided to take the test even if I weren't really ready. The key was I went in relaxed. I expected to be ready for the next test, I looked at the current test as prep work for the real thing.
As I sat there, a teen sat next to me and took his tech test and left me chugging through my test. He passed, I thought happy thoughts for him. Twenty minutes later I handed in my test. Fifteen minutes later I was given a thumbs up.
Of course I grabbed an amateur frequency chart and cackled evilly; "Mine, mine, it's all mine." but that is another story.
The point is you can do it. There are free on-line tests which do help. Take them.