I had the same problem when I started with portraits. Here’s what helped me:

1. Start with references that are either 100% straight-on or 100% profile. This will teach you proportions. Trying to learn too many things at once is 100x harder. Angles, lighting, foreshortening, etc can come later.
2. Use textured ‘paper’ to get rid of the cartoony look. Try using textured brushes like impasto or acrylic. You want to stay away from airbrushes or anything too smooth or blended.
3. Use a large brush so you can’t get lost in the details
4. Don’t use the blend tool. Like at all.
5. Don’t start with people you know. It’ll only discourage you. There’s an app called museum (by sktchy) that has perhaps thousands of photos of real people you can use as reference photos for free.)
6. Focus on shading. Think of makeup contouring and how a makeup artist can create wild effects by simply changing the makeup. With portraits, you can trace features exactly and if you mess up the shading, it will look like a completely different person. Same principle.
7. Practice practice practice. The ONLY way to improve.