"Most Catholics use contraceptives and loads have abortions... surely none of them can really believe the Catholic Church has religious authority over them or they would do everything the Church says? Or do they just believe they'll be forgiven? If so can't they do whatever they want and be forgiven?"

Is that not a major part of the Catholic religion in a nutshell? It is a big part of Catholic teaching that you can behave in the most vile way imaginable. As long as you trot off afterwards and tell a priest that you are sorry you are off the hook. The whole emphasis is upon you and your miserable soul, any victims, or bad consequences that are caused by your actions are irrelevant. It is hardly surprising that this kind of thinking leads to all kinds of unpleasant behaviour.