Failing one part is not failing the CBT
If you have just opened a result and found that you failed a part, the first thing worth knowing is that you have not failed the CBT. You have failed one half of it, and you resit that half.
The two parts are marked separately, so a fail on one leaves the other one passed and banked. If your numeracy went badly but the clinical part went fine, you resit numeracy. You do not sit the hundred clinical questions again. Plenty of people read the letter and assume they are starting from nothing, and they spend a week revising material they have already proved they know.
You get up to three attempts on one application. There has to be at least ten days between attempts, and you get a different version of the paper each time, so resitting is not a matter of remembering what came up. If the third attempt does not go your way the application closes and there is a six month wait before you can apply again. Six months is a long time and it is the reason not to rush the second attempt to get it over with. Ten days is the minimum gap, not a recommendation. If you need three weeks, take three weeks.
The ten day gap is the part people misuse most. It is enough time to fix something specific and it is not enough time to revise everything again. So do not revise everything again. If numeracy is what went wrong, work out which kind of calculation went wrong. Unit conversion, weight based dosing, infusion rates and fluid balance are different skills that all look like numeracy from the outside, and most people who fail the part are not weak at all four. Find the one, drill it, and sit some timed sets so that thirty minutes stops feeling short.
It is worth saying plainly that a resit is not a mark against you professionally. The NMC treats the Test of Competence as a standard to be met, not a competition, and nothing about your registration records how many attempts it took. Employers who sponsor overseas nurses are used to it. The number of people in your position is large and the reason you feel alone in it is that nobody posts about the attempt that did not work.
The practical advice, then. Read what the letter actually says about which part you failed. Resit only that part. Take more than ten days if you need them, because the six month wall behind the third attempt is the thing to be careful about, not the ten day floor in front of the second. And use the gap on the specific thing that went wrong rather than on a general sweep of everything, because a general sweep is what most second attempts are, and it is why some of them go the same way as the first.