In many cases, the verb is automatically reflexive in English (as far as I can understand, in Polish you add a reflexive pronoun?). When we say "I am shaving", the immediate understanding is that it is done to ourselves. Naturally, for men we assume the face, and for ladies the legs. Now here's the gold: "Myself" can be used, euphemistically, to mean your genitals. That's the first image that comes to a listener's mind. Aside from that, it could mean your whole body is shaved.

"I'm shaving myself." is grammatically correct. However, you would probably only use it in a situation where somebody else might shave you. Barber: 'We're done with the haircut. Would you like a shave, too?" "No thanks, I'll shave myself." In any other situations, you would just say "I'm shaving". "Are you ready?" "Just a second, I'm still shaving."

'I am shaving.' = usually refers to a man shaving his face. 'I am shaving myself' = refers to shaving the whole body like some swimmers do. I am shaving/waxing my legs= usually refers to a woman wanting to shave the hair from her legs. I know that you can say 'She is dressing myself' when you refer to a child that don't know yet how to get dressed. No. You say, " I am dressing the child myself." Or " She still can't dress herself."

"I am shaving": maybe you are shaving your owns; maybe your are shaving for others. "I am shaving myself": you are shaving your owns.