Speaking exams can be very daunting, but it's important to remember that these exams haven't been designed to trip you up in any way. The examiner will want you to do well too! 

It's also important not to be disheartened if you forget a word or phrase - it doesn't reflect badly on your Spanish abilities! We forget, mishear, and misinterpret things all the time in our first language without it impacting our confidence in our fluency, and it works the same way with other languages. 

Take a second to see if the word/phrase comes to you. If it doesn't, then talking around it is fine too, and can actually help demonstrate your fluency!

For example, if you were in the middle of your exam and forgot a particular word, then instead of switching to English, or just pausing, you could say "Me olvide la palabra, pero es..." and then define the word that you mean in Spanish. This can allow the examiner to fill in the word for you, and better mimics an actual conversation. It'll also give you a bit more time to collect your thoughts and get back on track.