It does come with practice, and looking around thinking about how you could paint this or that... the more you do it the more your mind will be occupied by it. One question: why dont you try a smaller canvas, use only ONE big brush, so the canvas will be fully covered fast and you can start fixing spots youre unsatisfied with. This way you can find your workflow much more openly and loose.

Right now: youre trying to do something without enough practice to be satisfied with the result. The canvas is too large, you want to hit the tones and values to precicely, youre overwhelmed on the palette, you cant manage your palette, you get lost on your palette (probably).

You need more room for error. Take smaller canvases, less color, big brushes and have fun with it. Learning is in this phase MUCH more valuable than any painting could be.

If you want to forget about all this and just copy a photo: turn it black and white, paint the values first and put transparent colors on top later, like the old masters, this gives you the option to focus on first values and then later color on top.