you need to breathe properly

Your voice is not stable due to likely incorrect breathing. You might focus on materials with the short sentence, or break long sentence into short ones. Keeping your voice stable might be your first priority.

There is no degree for us to earn, so as long as we feel confortable, we work on one aspect of the spoken English. Intonation and stress might come back to you when you practice some long/compound English words. We all run in a spirak model. We improve one aspect and feel good to move to another one. After a cycle, we feel we need to come back to improve some basic aspect again to get an overall improvement. For example, after many years, I feel to move into the next level, I need to train myself to pronounce some very basic sounds and words to have a habit of holding correct position of mouth, tongue and lips.

You are your own judge.