What's the main focus when you're sparring in mma?

The point of sparring in any martial art is to put everything together that you have been learning and training separately and to do so in a free-format as oppose to a set drill where you know what is coming and what you are going to do. In a real fight, you rarely ever know what is coming next and you cannot be static in your response.

Sparring used to develop and test many things such as your ability to change with the ebb and flow of the "fight" against an adversary who is also adjusting to you and your techniques.

You are trying to hone many skills - both the physical and the mental aspects. For the physical portion, you are testing your techniques, developing timing and range judgement. On the mental side, you are trying to develop a quickness of mind and a mindset such that your actions are not controlled by your ego and emotions. This are just a few examples of many things you are trying to train in sparring.

Sparring or actually fighting because the two are really different in some ways. Sparring is not like real fighting in the sense that you are not trying so much to bring about the physical destruction of your opponent like you are trying to do when fighting. Instead you are usually working on just further developing some aspects of your arsenal of skills. That and it also is a way for a fighter who is training to monitor their skills and ability and where they are in their conditioning and keeping those skills sharp or developing them further and more fully. A fighter should approach sparring as an opportunity to learn really is how I always approached it myself and with those fighters that I trained. Come fight time its pretty much too late to do any of that and instead you are doing those things that you have been doing in sparring generally with bad intentions and for the purpose of bringing about the physical destruction of your opponent.

Briefly I dont want to hurt your idea but you seriously need to think of centered instructor.

Fight hard