Unfortunately, every pupil only gets one attempt at an 11+ exam. There are scenarios in which the pupil takes multiple 11+ exams, but they will never be the same. For instance, you might need to take several exams when you’ve applied to multiple schools, especially schools outside of your area. However, you will never be able to take the exact same exam twice to avoid any unfair advantages.
Having only one attempt does add up to your stress levels. But keep in mind that this will be the same for everyone levelling out the playing field. It's very normal to feel a bit stressed. This simply means that you're taking it serious. Give it your best shot and don't think that the rest of your educational and professional career all hangs on this one exam. That's certainly not the case.
At some grammar schools there's also the chance to take the 12+ or 13+ late transfer test. This exam is focussed on determining which new students can be offered a place in Year 8 or Year 9. Students might before much better after an additional 18-24 months of age and score much better here than the 11+ exam. Whether it is possible to do a 13+ exam really depends on the school and the year, so I do recommend checking in with the school whether there's an opportunity if you didn't make it wit the 11+ exam.
Why not give the 11+ courses a go? There're for free!