Colour is distracting. A bright red jumper and a deep green shadow can be exactly the same lightness, and while you can see the colour difference easily, you will happily draw them as different values and wreck the form.
Switch your reference to greyscale and the truth appears: broad areas of light, broad areas of shadow, and a small number of accents.
Do a thumbnail first — a study no bigger than a playing card, using only three values: light, mid and dark. If the thumbnail reads clearly at that size, the full drawing will work.
Raise the contrast on the reference while you study it. Pushing contrast groups the values into clear families and shows you where the real shadow shape begins.
Then draw the shadow shape as one shape, not as shading. Almost every convincing drawing is built from correctly shaped shadow masses, with detail added only where the eye is meant to rest.
When you go back to colour, keep the value study beside you. If the colours match the values you found, the picture holds together.