There is one method which i recommend, which helped me to become comfortable and more fluent in French. That is to read plays. Even though plays may be out of date with regard to current slang, nevertheless, the fact that the entire work is dialog helps the reader to get a sense of the the rhythm of the language. I would suggest doing a web search for "Playwright+modern+American" and "Playwright+modern+British." That way, you could find some titles of plays. So, for example, if you did your search and found a list of American playwrights, you might see Arthur Miller and Death of a Salesman
. Then you could search online for a text of the play, and read it without being obliged to buy it or find it at the library.
This page at Bibliomania
has links to the full texts of several plays. A few (Ibsen and Molière) were not written in English, so they may not help. The others are Elizabethan and Jacobean (Shakespeare and his contemporaries), and the rest are Restoration playwrights--which means playwrights after 1660. For Shakespeare and the Elizabethans and Jacobeans, the language may be difficult. We have already gotten to the point where the English they spoke, which is modern English, is becoming difficult for modern English speakers. But the Restoration playwrights should be understandable for you.
The link leads you to texts of plays which will be difficult for you to ready and understand, which is why my first advice is to find plays by modern American and British (and Irish--they are very good) playwrights. But the plays on the page i linked above are many of them by authors who helped to mold the English language we speak today--if you can find you way through those plays, you will have taken a swim in the last 400+ years of the English language.