Hello DILO. It is indeed a good idea to start with gray scale if you're having trouble with values. It looks like you did a good job of making the shading basically consistent. But it looks very dull because you only have two shades of gray that are very close in value, which isn't outright wrong, but most images need a much higher level of contrast to be effective. I suggest using a greater range of values or increasing the contrast between the two you already have (I.E make the light parts lighter and the dark parts darker). If you want more realism, you'll definitely want to add more values; at least one each for light, midtone, and shadow; or one for highlight, light, midtone, shadow, reflected light, and so on, if you want to go all the w ay.
Now here's the tricky thing. Even if I could tell you everything that you did wrong, I think it wouldn't help you much. The reason for that is you said you feel like you're getting nowhere, which makes me think you need a good book or other resource to give you some guidance. This is one book that I found helpful, and it's free:
https://archive.org/details/Andrew_Loomi...ul_Drawing
The section about lighting and values starts at around the middle of the book. The description of highlights is not ~completely~ technically correct but, overall, it's a very good summary of the basic principles of depicting values. I also suggest reading this page for a more technically thorough summary:
http://www.huevaluechroma.com/021.php
Let me know if you have any questions.