You know, that could be part of your problem---and the reason your arm gets sore after pitching. That arm slot might not be the most comfortable for you. Would you consider dropping down a bit, to a straight 3/4, neither high nor low? That would probably take some of the pressure off your arm and shoulder, and at that angle you might not have to think so much about your mechanics and concentrate more on just throwing the ball. Also, you really need a good changeup, and I would suggest you try a palmball (what is often referred to as the "Bugs Bunny" change). That is one of the easiest on your arm, because you throw it with a fastball motion; you grip it with all four fingers on top and the thumb underneath, well back in the palm of your hand---but don't grip it too tightly, because you don't want to squeeze the juice out of it! And the most important thing is to relax, let your arm lead the way and the rest of your body catch up with it. Too many pitchers do it bass-ackwards, and therein lies the problem.
You might even consider getting closer to the sidearm delivery, which is by far the easiest on the arm and the shoulder. Maybe hyou'd lose a little speed but would gain much better control and command. Think about these things, give them a try and see if one or the other might work for you.