With what you've described, you've obviously gone through quite a bit in pursuing this goal. It's clearly important to you, and I think you recognize that giving up wouldn't really be what you want. So I'll say it: "Don't give up!!!"
My recommendation is to study (not just "read") Bruce Eckel's
Thinking in Java. It's available free as a download from the source...
http://www.mindview.net/Books/TIJ/ Or as an online reference...
http://www.faqs.org/docs/think_java/TIJ3.htm Or as a hardcopy book.
The reason I recommend this text is that it delivers what the title implies: You will begin to "think" in Java. So if you're having trouble retaining bits and pieces from sporadic studying, this might be what you need to lay a solid foundation. When the pieces actually come together, they will be easier to retain. It will take time -- especially working through the exercies -- but it will pay off. When I first started learning Java, I struggled with some other texts, but the lightbulbs really started coming on with
Thinking in Java. This book will go a long way in preparing you for the SCJP exam, but it will not get you all the way there. For that, you will need a text specific to the exam. But once you have that solid foundation, the
test preparation won't be as difficult.
And along the way, keep posting at JavaRanch.