You have a good foundation with your art. I've been using Photoshop for many years. There are usually trial versions of software available to download. I am biased, but I believe that the software does make a difference and can add polish to your work. If your serious I think its worth it to at least try out some higher end programs. (If your budget allows) It's worth making the investment at some point.
I know there were some Photoshop tips and tricks books that were really helpful to me back in the day.
Now everything is online, so much more convenient. In the end it is still your skill and talent that will make the work shine, but the right software can help to make a good thing great. There are so many great tutorials online. I love watching speed drawing on youtube. It really helps to see how other artists use their tools.