“Everyone who has made a success of watercolour has made mistakes in the early stages, they are a natural part of learning a difficult medium, so don’t be discouraged” (John Wilkinson in ’The Artists’ Guide’ Pub. Dorling Kindersley).
It is certainly true that to become proficient in any medium constant practice is necessary and that mistakes will be made. I would go further and say no matter how proficient one becomes, mistakes will continue to be made, especially if one wishes to develop and regularly experiment in order to strive to learn new skills.
Although corrections or improvements can be made in watercolour (by lifting, washing out, using other media over a watercolour – eg ink, gouache, pastel or acrylics) it is probably true to say that corrections are easier in acrylics and oils. In acrylics areas can be painted over with gesso and then repainted, or just repainted in opaque colours. In oils, colours can be similarly painted over when wet or dry, or be scraped off with a palette knife. A coloured glaze can be floated over a dry oil painting and, if not liked, instantly removed with turpentine, without disturbing the dry under painting. Acrylic glazes will dry much more quickly than oil glazes, but can be removed with water. Whilst oil glazes can be applied over an acrylic painting, acrylic glazes cannot be applied over an oil painting.
So, as Wilkinson said, don’t be discouraged by your mistakes; better to regard them as stepping stones on the road to success and to remember that there are lots of resources out there which will help you to become more proficient and hence more successful.