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.