I found that I tend to get backlash if I start with too ambitious of a goal. Instead, I'll aim for something stupidly easy and just focus on getting started at all. I've found that works much better.

At first I aimed for 1 hour of exercise a day. That failed within a week. So then I aimed for 10 minutes a day, which barely felt like exercise, but it got me to the starting line consistently! The trick for me was treating anything past 10 minutes as beating expectations, rather than treating anything under 1 hour as falling short. In practice it means I'm able to get about 20 minutes per day consistently, which is still much better than 1 hour never.

The key for me was making it so I could consistently feel good about a reasonable goal instead of constantly feeling like I was falling short of my own expectations.

Now if only I could apply that to flossing.