Hey!
It's totally normal to feel stressed - live interviews typically feel more daunting, but I'd say they are generally a nicer experience than VIs as you are able to genuinely build a rapport and speak with the interviewer (and they are sometimes more conversational as a result). The interviewers won't be trying to catch you out - they're simply trying to get a sense of how you think, how you communicate, and whether you'd be someone that they'd enjoy working with day-to-day.
In terms of preparation, it really helped me to get comfortable with the core story that I was telling. Make sure that you are clear on why you are interested in working in commercial law (or law more generally), why you wish to work at that firm, and what draws you to their training contract. Additionally, I would create a 'bank' of competency examples from your previous experiences so that you have them in the back of your mind - for example, times that you have worked in a team, handled pressure, or taken responsibility. This gives you something to fall back on, even if the question isn't phrased in the way that you expected. I also found that practising my answers for the more common questions out loud really helped to calm my nerves and helped me feel prepared, even if it was a little awkward at first!
On the day, remember that it is completely okay to pause and think before answering. Silence is not a bad thing, and you won't be expected to respond instantly - it's usually more effective to give yourself a few seconds to think before jumping into an answer. If a question isn't clear, it's also perfectly acceptable to ask for clarification to ensure that you answer the question in the way that they are looking for. It sounds silly, but I'd also try to engage with the interviewers as people - maintaining eye contact (and as someone who is neurodivergent, I know this can be tricky at times!), listening properly, and responding naturally can go a long way. I'd also encourage you to show genuine interest in the interviewers and their careers as well - you could ask them questions on their work and practice areas at the end of the interview, which will show them that you genuinely want to learn more about the firm.
It helps to try to shift how you frame the interview in your mind - try to treat it like a two-way conversation rather than a performance. They are assessing whether you'd be a good colleague just as much as whether you can answer questions well. Almost everyone is nervous during interviews, even if it doesn't look like it! A partner during one of my vacation schemes said that it is a good thing to be nervous before an interview, as it shows that you care - I'd try to use it to your advantage!
Overall, I'd just encourage you to be yourself and show your motivations for working at the firm. You have clearly impressed them already to be invited to the interview stage, so try to remain confident in your own abilities and I'm sure you'll smash it! Best of luck!!