I thought I'll point you to online resources available for Queueing Theory. Hope this helps!
The first one and the second one consisting of a list of books available online.
You could also get motivation (at the cost of missing some finer mathematical details) from here. The solutions are also available here
If you master pre-algebra, then you can figure out almost any other branch of mathematics using the appropriate study material. Geometric formulas will be second nature to you. Trigonometry and Calculus are not required to graduate from every high school. If you are strong in Algebra, then your college placements scores will exempt you from college preparatory courses.
College preparatory courses are great if you want to master the fundamentals. I suggest you take all the college preparatory courses in your field if you are going to specialize. For Mathematics, you should take discrete mathematics.
Because of the way the brain works, you will gain better dominion of subject matter by studying for a few hours everyday rather than cramming. Yet, you seem to have found out how some high school experiences are less adequate than independent studies.
You might want to go for the General Education Development test, then transfer to a college or university. A community college offers more advantages for students. You can get an associates in arts degree and transfer to a university from there to get a four-year college degree and postgraduate degrees. (You will need to pass college algebra and another college level mathematics course to get your associate in arts degree.)
Have you ever skimmed or read from a GED preparation workbook? You should go a college library and take it out. It's similar to the SAT workbooks. These books will give you detailed explanations. Yet, what do you mean by progressive practice problems? The word progressive can have many meanings; do you mean updated versions? That's up to the student to send in suggestions and report errors to the publishing company.
Remember, it's what you learn that counts. Most things we believe to be requisites are psychological exaggerations. Remember, being a student is a profession.
Best Answer
Why not take a look on the sections on model theory in the reading Guide on math logic which you can download from http://www.logicmatters.net/tyl ?
Yes, Marker's text is standard; but it is tough. The Guide suggests a few texts that bridge the gap between the entry-level model theory in standard first math logic courses and the advanced text by Marker. The headline news is that Hodges's Shorter Model Theory is a particularly good intermediate choice: but there is a lot more said in the Guide.