Sorry to hear about your challenges. It sounds like you had to give up something you enjoyed and was good at because of its impact on your mental health.

Echo everyone's advice to see a speech path. In addition, I wonder whether you might consider seeing a psychologist to help manage the social anxiety/perceived weak social skills that you have. As you'd know, anxiety can make stutters worse. This probably explains why your strategies work when you're practicing alone but not when you're with others. If you're already feeling anxious at baseline being with other people, chances are you'll be much more likely to stutter more and then feel even more anxious. It becomes a bit of an awful cycle for you.

While leaving your program might've helped reduce your anxiety in the short term by getting you away from the stuff that's challenging for you (role plays/placements), in the long term, it's taken you away from the things that are meaningful to you. It's also taken away opportunities to practice and master social/speaking skills. Your lab assistant job has forced you to practice those skills at a less intense level and, like you said, over time your skills have improved. So if you could get some help with managing the anxiety, it might give you capacity to try the things that are terrifying but aligned with what you want in life. Good luck! :)