Of course it is possible to teach yourself fly fishing. There are many fly fishers who have never attended a casting course or taken part in a guiding. This decision is up to each individual.
In our view, however, correctly learning the basics of fly fishing from A-Z is extremely beneficial. Generally, it is very difficult to correct errors in technique once the body has learned them, and these errors can lead to further difficulties that make it hard to acquire new casting techniques (trick casting).