A phobia is an unreasonable fear, often incapacitating. It may have its roots in a previous experience of which you are reminded because of similarities to the present situation. In common language, you are replaying an old tape. You are pulling up from the subconscious the emotions of a previous experience, warning, threat or whatever that at one time elicited an emotional response from you.
Phobias vary in their severity.
Most are quite easy to fix. Some, like agoraphobia, can be more difficult. Basically what happens when a phobia takes over is the child or younger person within you takes charge. The actual memory from which the emotions arise usually does not appear, but the emotions surely do. Take women's fear of a mouse, for example.