You sound a lot like me, which drove me to log in and reply. Here's what works for me these days and tbh I'm quite pleased with the results. I will admit that the circumstances under which I started this I felt very motivated to complete this.

Here's my advice, it's simple. Keep a streak in Duolingo, it'll remind you. I like the Android widget too. I do maybe one minute of work daily to keep my streak and at least for me it's the cornerstone habit I've then added other stuff over time. Once I had a small streak built up I went to the gym every day to either walk or run and when I ran I just did exactly what the couch to 5k app told me to do. When I was walking, I did duolingo. Self reinforcing.

Being able to run 3 5k's a week by the last week I no longer have problems basically exactly like you described. Or if I do, it's not nearly as egregious as it once was. The only trick for me at least was to just do exactly what I was told. It ramps up and I just never stopped before I was told to.

Key takeaway is that your body and brain are malleable and will adapt. Hope this or something like it helps!