I think those areas you mentioned are the same areas where most of us struggled at first. Try to maintain the correct angle as AFDavis11 stated. If you stretch the skin on your jaw line to raise above the line you will find it easier to get that area smooth. Like you are shaving on a flat surface vs. a ridge. With your neck pay close attention to the direction of the growth and take your time. My neck is sensitive and I don't go against the grain. By reducing the beard with a with the grain pass and then cross grain passes it works out well for me.
I think you will find that with time you will be able to figure out what works best for you. Don't give up...
"It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt." - Mark Twain