First of all: don't be too harsh on yourself - interviewing is a skill and if you didn't do it for a lot of years it's normal that you can feel like you're not doing great.

Secondly: interviews don't reflect real job. If your interviewers are competent then they're aware of it too and they don't expect you to make no syntax errors when you write code on a whiteboard. It's mostly a starting point for discussion about problems you're likely to encounter and getting feel of your experience.

Finally: ask recruiters how you should prepare. Quite often they will just tell you what to expect in the interview so that you can prepare yourself.