How can someone overcome writer's block and continue writing a novel that has been stuck for years?

I've not had writer's block so take my answer with a grain of salt. I've also not had a novel stuck for more than a few days…

I can only talk from my limited experience but I did get uneasy about my progress at one stage and realised I was trying to get too much into one story. In the end, I found that I could end the story effectively at an earlier point and use most of the bit I cut in a second book.

I have also found that my own emotional state and confidence play a part. I have found it worthwhile reviewing my motivation for writing and remembering that I simply enjoy writing a story. Nobody may ever read it but that's okay for me.

Another strategy I used in my first book was to write the ending. I found it gave me a pathway. I didn't use that particular ending in the end but it helped me focus.

My final thought is that it seems to me that one can easily write too much… try to include too much… the end result being things get clogged - like plumbing. Perhaps, thin things out. Skip some dialogue, lose a support scene, shorten a large explanation to something much simpler. In my second book, I am starting a different type of government. It has taken a lot of writing that I have ended up tossing out - who wants to read an explanation of politics??? Instead, I have a short dialogue where the basic idea is put forward and then two short scenes where a simple constitution is shown (380 words) and a description of the first selection process (400 words). The reader has a lot of power to take the scaffolding words and fill in the life.

Good luck with your writing!