“You have to improve structure of the speech, also your body language, and vocal variety, and…” Have you ever felt overwhelmed by suggestions for improvement—clearly, well intentioned? They provide a good map of improvements. You should tackle them one at a time, by focusing on one thing to be improved and letting other things be as they are.
Want to improve vocal variety? Drop the speech writing and read a famous speech or letter. Interpretive reading is a great opportunity to practice your vocal skills and it is fun. Moreover, you could find many examples to learn from. My favourite is Benedict Cumberbatch reading Sol LeWitt’s letter to Eva Hesse, what is yours?
Want to improve speech structure and content? Guineapig it on colleagues, family, your dog! Don’t worry much about other features, like posture and vocal variety. Open Mic is a great model for testing material, which is used by experienced speakers.
Want to pump your impromptu speaking? Volunteer for Table Topic Sessions at Toastmasters Club Meeting, they are often open to guests, or attend (run?) a Table Topic Marathon. Your learning goal is to start speaking on the spot, so practice this and only this.
Want to give a helpful speech evaluation? Limit your suggestions to 1 or 2 points maximum. I prefer “3-2-1” evaluation scheme: three observations on achievements, two suggestions for improvement, one takeaway.
Practice makes you better. Just keep going. One step at a time.