The English language is a stress timed language, which means that the sentences have stressed words and words that are not stressed. The sounds in these words that are not stressed tend to blend together, and the following sections look at how the different sounds in words in a sentence react to each other.
To improve fluency, you need to understand how words merge together, and become comfortable with the rhythm of the language.
Some languages, especially those which are syllable timed like Spanish, have a regular beat, which is related to the syllables, but English doesn't have that. The rhythm of English is controlled by the stressed words, and the other words just merge together in-between the stressed words.