I have problems with comprehending sentences. Many times what I understand after reading a sentence or sentences is totally different from what it actually means. I can understand the meaning of each word, but as a whole it's difficult to get a sense of the sentence. It makes it difficult for me to explain what I read to others in my native language.
Failure to understand what you read may be caused either by reader factors or text factors or environmental factors. Often a combination of two or more factors is the reason for misunderstanding.
Reader factors include proficiency in the language of the text, especially vocabulary and grammar, background knowledge and interest in the topic of the text, and alertness (for example, not reading while tired or distracted).
Text factors include clarity of expression, syntactic complexity, and vocabulary (more or less common words).
Environmental factors include the place where reading is done (for example a quiet study vs. a busy coffee shop).
A common cause of failing to understand is the inability to spot idioms. A literal reading of the text is likely to cause confusion. Another problem is the polysemic nature of much English vocabulary. As a simple example, if you only know while as meaning during the time that, then you may be confused by this recent tweet: While I had a great meeting with NATO, raising vast amounts of money, I had an even better meeting with Vladimir Putin of Russia.
Perhaps you could include in your question a couple of examples of when it turned out that your understanding was 'totally different' from the actual meaning.