Cutting my own hair - how to get around the ears

I can almost cut my own hair just how I like it.... but there's one problem. I can't get the hair around my ears done right.

I always use a #12 clipper attachment on the top, sides and back of my head.

For around the ears I use a special attachment.

The problem I have is that the hair around my ears grows in over my ears too soon.

My question is, do I need to thin out the hair on my sides before trimming around the ears?

Is it even possible to get it done right by doing it on my own?