我想讲中国普通话

我的母语是西班牙语，我正在自学中文，

我的母语是西班牙语。(现在)我正在自学中文。They are two whole sentences, so you shouldn't have used "," between them. Instead, you have to use "。" between two sentences when there is no conjunction between them.

我觉得中文是一门很难学习的语言。
尤其是它的发音很难，

我觉得中文是一种很难学习的语言，尤其是它的发音（很难）。

1 “一门”mostly is used before "subject(学科)".

2 Because you used "尤其" in "尤其是它的发音很难", it is not a whole sentence, and "我也正在学习写中文" is not proper to be used behind it to structure a whole sentence, so you cannot use "。" between it and the last sentence.

3 When we use adjective to describe something, and use "尤其" right behind that sentence to further empharsis that a part of it is "adjective", we can omit the adjective in the clause.

我也正在学习写中文，

另外我也正在学习中文写作。When the topic is different but similar with the last topic, we should use 另外 to make it sounds natural. It is just like that in English we often use "Moreover", "Furthermore" and so on in these situations. 中文写作 means Chinese writing.

我想努力学习中文，以后应该可以说的很流利。

我想，只要我努力学习中文，以后应该可以说得很流利。or 我会努力学习中文，以后应该可以说得很流利。The first sentence means that "I think I will speak Chinese fluently in future as long as I study it hard". The second sentence means that "I will study Chinese hard, and I may speak it fluently in future."

1 只要 means "as long as".

2 "我会努力学习中文" is to describe your willing or your decision (It is something you will do), but "我想努力学中文" is just a thought (It is something you may do).

3 We often use structure as "说得流利" in sentence. It is "verb+得+adjective". In this structure, the adjective is the complement of the verb. When "的" and adjective are put together, 的 can only be put after adjective to complete the adjective or make it a noun. For example: 美丽---》美丽的。  我有一条美丽的裙子。 比起丑的，我喜欢美丽的。   

我想讲中国普通话

我的母语是西班牙语，我正在自学中文，我觉得中文是一门很难学习的语言。
尤其是它的发音很难，我也正在学习写中文，我想努力学习中文，以后应该可以说的很(更)流利。