學外語是一項就像接觸的性運動
-There is no indefinite article in Mandarin. We use "one" only while emphasizing the number.
-Do you mean "contact sport"? I couldn't find the general-used term in Mandarin so I translated it myself.
-I personally prefer simile than metaphor here.

-Same as I mentioned before, there is no indefinite article and the number here is not necessary.

學習外語不像學一個門科目，而像學一項運動。
-The measure word is crazy in Mandarin haha
-To be honest, I can't explain why it's better to keep the "a" here. It just is.

學外語是一項就像接觸的性運動。
像摔跤或者美式足球一樣要穿保護用具。
學外語的保護用具是你的臉皮。
-Nice analogy :)
因為犯錯是不能不可/無法避免的，所以你臉皮要厚一點。

學習外語也像學習拉小提琴一樣：不管多麼不好聽, 你都要繼續練習。
你先要大膽犯錯，然后後得到反饋回饋，再然后後就能說得好一點更好。

-These (blue) parts are not absolutely wrong. I just edited it to be more natural to me.
-I noticed that you use traditional Chinese, so I use 後 instead of 后. 反饋 is a term used more often in China. In Taiwan we use 回饋.