The best way to learn anything (also deep learning), especially for beginners, tends to be IMO to follow some structured approach and stick to it. So pick a good book or course and try to finish it (or the fundamental parts). It may take a bit longer to see progress than randomly following youtobe tutorials or notebooks, but builds a solid base that makes learning in the mid/long term more efficient.

The most plausible explanation is overfitting. How do they compare in terms of error in the train set?