How to maintain focus during tests?

Hi all, I’ve failed a test because despite knowing the answer I’ve given the wrong one, and this is something that happens to me from time to time in various situations. I suppose it’s a lack of focus, but don’t know if it’s the only problem. What are your ways to cope with this? Right now I’m a full time worker getting a second degree in CS (first one was in Electrical Engineering). Thank you!