Hi Jen, as a fellow introvert myself, I would say you are not alone! There are definitely days where I feel like my lack of assertiveness have left doctors or patients to not listen or care about what I recommend.
Honestly, I think it all comes down to how you want the patient's outcome to be. If you in your heart know what is best for the patient, then I know for a fact you can overcome that obstacle of being shy and advocate for what you practiced so darn hard for!
For me, what helped the most was talking about how to open up more to coworkers whom I trust won't judge me. I've also forced myself to attend socials just to get out of my shell. I won't say I am 100% open to talking to anyone freely just yet, but I've improved from when I started.
It is easier said than done, but I promise with consistency and the mindset of knowing that this is for your benefit... I believe this is where you belong!
also p.s. if people judge you for not opening up, then that's their problem because they have nothing else to focus on LOL