The reason you must have your own IP address when you want to use your own SSL certificate (when you do not want to use the server shared certificate) is because of the way SSL and the webserver works.
For shared hosting (when many domains are on one IP) the web browser will pass the name of the domain being requested inside the httpd headers along with the request. This way, apache knows which domain you are trying to access even though there are many domains on that one IP address.
When the same request is made via SSL connection, the connection has to be made before the request can be sent. That is the connection when the certificate is passed. Before the connection is bade, the only information that apache knows is which IP the connection is being made to. It has to be able to know which certificate to send before the request is made, thus you can’t use mutiple certificates on the same IP.