You can read and write fluently because you don´t feel compelled to rush. When you are speaking - especially with native speakers of the language - you feel obliged to keep a certain pace. You instinctively intuit the rhythm of the conversation and attempt to match it.
As others have stated, talk to people all the time in English.
But in addition to that, anytime you are alone, *talk to yourself* in English. If you're home, in your office, driving your car; wherever you are, talk to yourself. Talking by yourself gives you the opportunity to make mistakes and work on your diction, speech, and confidence.
If you do that for a few weeks, your ability to converse easily and confidently with others will greatly improve.
EDIT: Another thing I just thought of that I do with Spanish: If your work has any redundant tasks or logging, keep your notes in English. For example, I perform the same steps each time I use GitHub, so the steps are in Spanish. When I perform these steps, I recite them out loud in Spanish. Also, even though all of my emails and work with others is in English, I keep my notes in Spanish. This keeps me thinking and talking all the time in Spanish.