Great postings by both Coach45 and baseballbum.
If you have some video of you throwing from the mound, please post it. That way people of knowledge on this web site will be able to literally watch your mechanics and offer relevant information/suggestions.
If you have no video, have someone tape you from the front, back and "open" side and then post each. Since you're already experiencing "arm strain/fatigue", remember that you don't need to throw hard or throw breaking balls. Just giving this forum a chance to watch your mechanics should go a long way toward helping you.
Also, be sure to answer the questions asked by Coach45. Your responses to each may be significant.