Disclaimer: I'm just a student, not yet a licensed SLP.

Here's my answer as a student: See a licensed SLP about this. Word finding difficulties can come from a number of different issues. It could be working memory as you said, but this issue can also sometimes be related to auditory processing deficits or others. A real assessment by a licensed clinician will help you pinpoint what's happening, and that will help you figure out the best steps to improvement.

Here's my answer as someone who has experienced similar issues personally: Try to do some activities that require you to exercise your improvisation skills. Story Cubes are one of my favorites. "Guess the word" games like Taboo or Catchphrase are also good because they require you to improvise a lot of language in getting the other person to guess the key word correctly. I did a little bit of improv theater in college and it helped a lot. I also found that learning a 2nd language was extraordinarily helpful with my fluency in English, which interestingly enough is now one of my main research interests going into grad school. :)