I agree you don't need to start over. It will catch up in no time. I use a single-blade safety razor to maintain my neckline. It works great. I threw everything else away.

Funny thing about a neckline is that as important as it seems at first, once you have a little bit of length it becomes less important, so keeping it perfectly straight or whatever won't be as important down the line. It's mainly a thing to communicate intent to grow a beard rather than slacker too lazy to shave. I am at about 2 knuckles depth on the chin now, so my neck line is really only visible if I throw my head back.