Nearly all of my career has been in embedded as well. OO didn't come up very much and it was mostly straight C and assembly. And lots and lots of electronics. It still love that most but it's not as practical as work as it once was. Although it may come back someday in "the internet of things".
The main thing is to not get discouraged. My only good quality is I will not quit, no matter what! Except if I lose interest, and then it's justified. You are probably the same way on your projects and you have to think of the ocajp as a project like that. I actually had to take it three times before I passed. The first time I just thought I would pass because I could write Java programs. I got 40% and I think the test was considerably easier then than it is now. So in figuring out WTH happened, I discovered this site and through it Entuware. I got 40% on the first Enthuware test. Then lots of studying later I took ocajp a second time but the style of the questions was now different from before, plus the test conditions were terrible, and I ended up failing by some tiny amount, 3 points, 1 point, I don't remember. I was so determined that I would have spent my last nickel and everything I could make selling pencils to take it until I passed it. I don't know how beneficial the certification is, but I wasn't going to let it get me. Leitner mode in Enthuware was really helpful to me. I had all the questions in the last two columns at one point. I think by the time you have done that, you'll be in a position to get 80+ on the Last Day Test. Very simple. The books don't cover the material in the same way, and I think it's good to use Enthuware itself as study material (in Leitner mode). If you have a question that extends from some test question then write some code to answer that. Most important, forbid yourself from making oversight mistakes on the tests. Change the way you think, or whatever it takes to do that.