Mercury represents your mind, your thoughts, and how you communicate. Mercury is basically what rules over your brain. When your Moon is in opposition to your Mercury, your mind and your heart are in two separate places. This aspect causes you not to be sure whether you should rely on your mind, your thoughts, your logic, and intelligence when it comes to making decisions, or if you should depend on your heart and emotions.
Moon opposite Mercury can cause you to doubt yourself and your decisions a lot. It can be challenging to know whether or not you are doing the right thing. It will be rare that your heart and your mind will work in unison with each other and agree that this is what you need to do. Usually, when you need to make decisions or take specific steps in life, there is a sense of inner conflict that you go through as you struggle to decide what side of yourself you need to listen to.
So, for example, if your Moon is in Sagittarius, your heart can want to crave freedom and independence. However, because your Mercury will be in Gemini you will desire connection, constant communication, and being involved with other people. The side of you that wants freedom will be at conflict with the side that wants to be connected with others, and it will cause a push and pull inside yourself.