Game design is something you do because you enjoy it. When you stop enjoying it, you should stop doing it.
Your regular job makes far more than game design ever will.
Staying motivated is central to successful game design.
You can quit at any time, or, more likely, just let things slide into nothingness. Almost every game designer will follow this path eventually. Some will follow it quickly. Be aware of this danger, and take steps to combat it. An unplanned exit from the hobby will mean all your projects effectively die.
Stay engaged with your designs, even if you're not doing something useful. Sometimes it's fun to make nice new icons, get some new art, just pore over the cards, or work on the game in some other way.
By jumping to a different project that's at an enjoyable stage, you can rekindle your interest. Note that this might mean starting a new project, and abandoning your current project. In reality, you may never come back to your current project, but you can decide that later.
Help other people with their games. This keeps you connected to design, and often, a design buddy will help to motivate you to work on your own game.
I like to do at least some game work every day. This is a pace sufficient to stop things drifting away.
Making a big change to your game means doing a lot of work rejigging everything. This can really sap morale. Try to just change the thing that's most broken. Having a week of game work ahead of you can be an onerous task you never manage to get over. And, once you lose track, it's hard to pick things up again.
Keep non-design work to a minimum. Things don't need to look good, or be exact. You can just fudge things during playtests.
My 21st game was an expansive project — the first medium-weight game, after 20 medium-light games. I put a huge amount of work into the first prototype. After two minutes of play, I realised the core activity of the game wasn't fun. I had to redo basically everything. The second, third, and fourth editions were the same. The game was not a rough shape with problems that needed to be fixed. I was still trying basic shapes. During the fifth revision, I couldn't force myself to work on the game any more. It was becoming too laborious. So, I picked up one of my earliest games — a completed game that needed my recently-acquired skills, in order to be made publishable. Now, I'm happily working on that again. Other people were interested in my 21st game, so I didn't tell them I'd abandoned it. A decision about it could come later.
I'll think much more carefully about designing a medium-weight game in future. This is why I stressed so strongly, earlier on, that you should design a very light game as your first game. You can either trust me, or learn this lesson for yourself.
This is also a good piece of life advice: gain a good understanding of your own motivations and typical behaviour. Don't try to change yourself. Just circumvent your negative behaviours.
A long period of disinterest or procrastination is a good sign that something needs to change.
If you haven't done a revision in a few weeks, accept that it's time to work on something else.
If you can keep game design fun, you can keep doing it. Otherwise, just quit.
The end of your game design career should be something you plan in advance, rather than the result of a long hiatus. Be honest that you will leave the hobby, and plan for it. I plan to go on to other things, several years from now. I figure that's how long my motivation will last, based on my knowledge of myself. I have some brand new projects planned before then, based on my best one or two ideas, and some loose ends from old projects to tie up. As I've said in previous articles, some games will be abandoned. This approach will mean that everything of significant worth gets out there into the world, and I have no regrets or unfinished business from my board game career. Everything worthwhile will be published, and everything unworthy will be deliberately ceremonially entombed, even if only in my mind.
Think honestly about yourself and your game design career. Realistically, when will it end? What needs to be done before then?
For most people, including you, board game design will sometimes be an intensely interesting process, and you'll be glad you did it. You should be realistic about your aptitude, and not expect success. By sheer numbers alone, It's unlikely you'll ever get published. It's like going on Survivor. You should try your best, but be honest with yourself that you're unlikely to win. The vast majority of designers who read this article will never have a published game. Like a designer's earlier, flawed games are just training for their later games, a board game design career can be seen as a life experience in the same way — not a success in its own right, but building a better person, for success in greater life projects in the future. Come to terms with this likely path in advance. You'll be able to walk away from an unpublished board game design career with the feeling that something was gained, rather than forgone.