先说你的问题：
was serious facing...不能用副词seriously
the value of .... is to practicing不定式或者是is practicing
应用不能是practice into，应该是apply the skills to actual projects 将能力应用到实际的项目中。
I analyzed the reason why I did not do my paper exam very well, I think I spend more time to practice the skills and not spend enough time to learn the the 这个well后是句号，必须分句，因为前后有两个动词。
i think.....the theory这一句最好用too...to 句型，然后是过去式spent，最好是完成时have spent
最后的逻辑关系不是但，应该是因此，所以不用but，用consequently, therefore, thus都可以作为结尾。
然后给你改一下：
事实上，我是非常认真对待这门学科的。我认为，我们学科的价值在于对代码的实际应用。我分析我这次final开始没有发挥得很好的原因，在于我将所有的重点都放在了 practice the skills，而没有很好的去熟记theory。但我依然认为这次final exam的成绩不能真实的反映出我对这门课程的掌握。
我的翻译：
In fact, I have treated this subject seriously. I believe that real application is what the subject is all about. After scrutinizing the reasons why I haven't exerted my full potential in the exam, I found that I put most of my efforts in practicing the skills rather than memorize the theories. Consequently, I don't think what my final grade have showed can accurately reflect my ability in this subject.
