You can't simply want it and hope it will happen and you can't put such standards for yourself for when you write your first symphony. It may not sound good at all, however you mustn't let that stop you from trying and trying again. Beethoven and Mozart both wrote tons of pieces, both good and horrible, before any of their first masterpieces were even thought of as simple musical ideas prior to their making. They spent years, even decades learning and mastering their craft before writing those pieces.
I'm saying that you can't hope to understand music and write down an entire symphony in one huge step. It takes time - years even, before a person has enough compositional experience and knowledge to write even a decent sounding symphony. If I were you, I wouldn't start with a symphony. I would start with mastering the basics like melodies, harmony and counterpoint. Once you have mastered those and laid your musical foundation, only then can you go on and build the fountain of ability you so desire.
Remember that Rome wasn't built in a day, and neither is musical ability. Learn to crawl on your hands and knees, then walk, then run, then jump. It's all a process and you can't skip any one of those steps no matter how much you want to. We all have to go through it, friend.
Good luck friend!