Hang in there Adrian. I would focus on the cheeks intil you feel really comfortable with straight shaving them. That will give you some more confidence in the neck and chin.

The chin is always harder to shave due to the curves and the courser hairs. It takes extra commitment and a sharp blade. Extra beard prep helps here. If you prepping the beard with a hot shower some have used shampoo and cream rinse on their beards to soften the hairs. If it's just a face wash, wash it twice.

The neck is hard to shave because we use our peripheral vision and it is almost out of sight. Have someone watch you shave your neck a few times. Explain the proper angle to them and have them mention when the angle gets out of range. Good straight razor shaving technique also involves a memory or feel of the blade in the difficult areas. This can only be improved by practice. Good luck.