Don't worry, most native speakers use simple words and structure in everyday conversation. More complex language is used in debate. The BBC have many good podcasts (the target audience is English) on varied topics which may help with hearing more complex language. More exposure in this way may help you to incorporate it into conversation. Good luck

Hi Sunny! I'm learning Spanish and I would consider myself at a high intermediate level. I feel like I have this issue as well, and I get frustrated when I think about a complex structure or word that I could have used, but I didn't. I don't know if there's really an easy solution for this, but I will tell you what I do. I have a small list of more complex vocabulary I want to use and I make a point to use it in conversation until it is a part of my natural lexicon. Really it comes down to practice.

I don't have an answer to your problem but I can say that simplicity is best unless you're in a situation that requires 'big words.' An example would be addressing a group of brain surgeons at a technical conference. Not only is brevity brilliance but so are simple words that get the job done.