My advice is practice , practice and still practice! The best thing would be to have the chance to speak with a native speaker regularly in order to improve your fluency ,and consequently your ability in constructing sentences, and of course your accent. There are loads of both english and american accents so choose the one you prefer and stick to it. For instance , I do love both but I decided to focus on the british accent so I listen mostly to british music and english podcasts , watch english tv series , watch british youtubers' videos and so on . Once you get the hang of an accent you can listen to materials in other accents . My advice is to focus on a specific accent and listen to material in that accent and not shift from one to another otherwise you'd get confused.
Don't get down or upset just because you haven't reached your goal yet. Learning a language isn't a walk in the park. It demands time , constancy and will. Everything else will fall into place.
Hope this helps ! :)
Break a leg.
I also struggle at the ability to create sentences in my target languages (Spanish and Korean). I think it is a matter of output vs. input. When we read or listen to someone speak in a non-native language our passive memory can recall the words meaning or we can use other words in the sentence to understand the meaning. To fix this problem, we need more output. We have to write and speak in longer sentences, which should help us become more fluent.
As for pronunciation, there are a number of English dialects that sound different. Decide on a dialect, either American or British English, and break down the syllables. Remember you have been practicing Chinese for over two decades, so it will take more time to become fluent. Focus on TV shows and pause after a sentence. Record each syllable and listen to your recording. Compare the recording to the TV show and if it sounds similar move past the syllable to the whole word. If it doesn't sound correct make those small adjustments.
Also find language partners that are not afraid to tell you what you are doing wrong. Italki has been great, but sometimes the people are too nice. I wish you the best in your English acquisition. I know if you study diligently, you can do it. :)