It think it's important to remember that there's not always a ton of evidence to support many of the "best practices" out there. Your child is very unlikely to be harmed by an occasional deviation from the "plan".
Cribsheet: A Data-Driven Guide to Better, More Relaxed Parenting, from Birth to Preschool
I also struggled with breastfeeding (for different reasons) and it had a major negative effect on my mental health. I wanted to stop, but I also felt like I'd be a failure and a bad mom. What helped the most with my guilt was reading Cribsheet by Emily Oster. There's an entire chapter dedicated to analyzing the studies that have been done on breastfeeding. If you can't or don't want to read the book there's a condensed version of her findings in this NYT article.