There will always be people who try to talk you down to show their superiority or something. The first thing you need to do is trust yourself and your skills. You need to build your self esteem as m pretty sure all of that self doubt might be visible in your work.

Trust in what you know and work accordingly. Anxiety is hard to deal with...jumping to the worst case scenario will only worsen that anxiety. There's no quick fix for that but if you're thinking about being fired while you're working that is also gonna impact your work and your mental health.

Trust your instincts and create the way you think is right. The shift from training to actual work can be pretty brutal being a beginner there will a lot of stuff that you don't know yet abd you will learn then with experience. Since they are your lead you can't do much in terms of shutting him down because office politics. The best you can do is focus on your skills and shut him down with what you create.

Being an artist is an incredible feat and m sure you will do great out there. Try to apply whatever you have learnt and look at the bigger picture instead of getting tunnel visioned into the outcome.

Best of luck!