You may well be plagued by a common phenomenon among amateur and profesional artists alike - memories of past failures. The painting you saw in your mind's eye at the start never really materialised and you were left with a desperate feeling that there is no point trying - you won't do it this time either.
It happens to us all.
Try to enjoy all the different stages of the work and keep the original thought in your mind. Planning is the key, so spend enough time on it, though I know it is difficult when you are keen to get started. Don't overwork a watercolour and keep the light in the painting.
Go away from the painting at intervals, make a coffee or something and return with fresh eyes. Take the painting to another side of the room and view from afar. It won't look as bad as you thought and mistakes will become obvious. Solutions come more quickly to a refreshed mind that a tired one.
Some people remove their spectacles to keep the work loose. It is difficult to work too tightly when you can't see properly and light and shade become more defined. The same effect can be obtained by half-closing the eyes to inspect the painting.
Leave the work on display for a day or two, observe it in different lights and spot appropriate changes to be made before you decide there is nothing more you can do to improve it.
It's what keeps us painting, after all, the need to improve or reproduce those we got right. The next one may be the best work yet, so keep trying.