We all have been there. It is tough. First of all, I suggest that you should stay connected to your core values and use them to navigate difficult situations. You have invested tons of energy into getting this offer and had motivation to do that. Before making the decision to quit, try the following approach:
Step 1. Limit your emotional exposure and fix the basics
The first thing you should do to limit your emotional exposure is to act.
Colleagues: stop engaging in doomsday conversations with toxic colleagues
Feedback: make it clear to your project leader that you will be better able to manage your stream when you have a clearer view into your performance
Productivity: add time blocks to your to-do-list, make temptations hard to reach (e.g. turn off social media alerts), start small
Step 2. Reframe
Step outside of your own perspective and consider the larger context of your situation. Remind yourself that the disruption you are facing, although unpleasant, is normal in the context of COVID, first year MBB experience, etc. And that everything related to the COVID has become a universal experience. When you remind yourself, that you are the one of many people going through such turmoil, it will be felt less personal.
Step 3. Refuel
Your network: renew your connections with your university network to overcome the fear of being alone after you quit
Your hobbies: even though COVID is limiting a lot of activities, you can start a new hobby on the weekend (e.g. digital drawing)
Sports: another way to boost your energy is to start with Nike Training Club home workouts
Food: my simple advice is to think about your diet: what you eat and when
Hope this helps!