How can I up my confidence level in my technical skills?
My advice is to focus less on the abstract unknowable questions "how good am I?", or "am I technically skilled?" - and instead focus on the concrete things which you've actually done. Focus on the achievements you made in your education. If you like, make a list of all the problems you solved and things you did in your PhD. When you get asked questions in your interviews, don't think about whether you are "good", just think about all the things you did and (metaphorically) give the interviewer that list, and let them judge for themselves if you are a good fit. The other clear solution to having nerves during interviews is simply to do a lot of interviews. Try to schedule some "easy" ones early, so that when you get an interview with a company you really want to work with, you have a bit of practice under your belt. You can also of course have mock practices with your colleagues and friends.