I highly recommend this book; it helped me immensely when I was learning to be more assertive at work. It shows you how to draw the line between pushover, aggressive, and assertive.
Nice Girls Don't Get the Corner Office
https://smile.amazon.com/Nice-Girls-Dont-Corner-Office/dp/1455546046/
We actually had a book club for women engineers at one of my assignments.
We read Nice Girls Don’t Get the Corner Office by Lois Frankel
As others have said, at this point it's best to look for a new job, where you will be compensated better.
A couple of resources that you might find useful:
1) Nice Girls Don't Get the Corner Office
2) A Power Mood
You are capable. You are worthy. You deserve to be paid generously.
http://www.amazon.com/Nice-Girls-Dont-Corner-Office/dp/1455546046
http://www.amazon.com/Lean-Women-Work-Will-Lead/dp/0385349947
If you haven't read these they may help.
edit: I saw your comment stating you only make 48k. Please read these books. They will help you. You are being taken advantage of.