As an ADHD writer, this mindset tip is such a struggle! Most of us know we need to be patient in order to see results, but it's so difficult to remember that in the moment.
We ADHDers often decide something is a failure if it isn't an instant success, but that's not how the writing world works.
Just because you hit a snag in the book you're writing doesn't mean you can't finish it. Writing a book—especially your first one—is challenging, so don't give up as soon as it stops feeling easy. Recognize that it's a process, and that even though it may feel like you're not making much headway in a given moment, if you keep working at it and don't give up, you'll get to the finish line.
And for those of you who have already finished your book but you're not seeing the success you want, remember that just because your novel wasn't a big hit overnight doesn't mean it's a bad book. It also doesn't mean it won't become more of a success later (this is especially true for indie authors).
If you're struggling because of not being able to get an agent, remember that just because your book was rejected by once—or four hundred times!—doesn't mean it's a bad book. It also doesn't mean no agent would be interested in it.
Just because your author career hasn't made any money yet doesn't mean it never will.
This same principle can apply to so many aspects of a writer's journey. So, when you hit a roadblock, stop, breathe, and really evaluate the situation as realistically as possible. Talk to a friend or relative and get their input. Pray about it. Then, once you've gotten a healthy perspective, decide where to go from there.