I have a chronic medical condition that sidelines me a lot. Although I am being treated, I have had to accept over the years that there are certain things that I just can't do (or might not be able to do, therefore I can't plan to do them and have people count on me) and someone else has to take up that slack. I know what I can do and I do those things well. Everything else, I expect someone else to step up, and they do, because I work with a great group of leaders who are willing to do their part. And I have no problem asking them to do things.
Sometimes I get ahead of myself, thinking I'm getting better, then realize it was just one of my good times. Sometimes I make plans I shouldn't. So even though I work at managing this I'm not perfect. I get frustrated with myself.
I hope you are asking for help. Professional help, for all aspects of your life that are affected, and in Scouts. Sounds to me like you need a good partner leader who can do the things you are unsure of. Working with other leaders has been one of the most enjoyable parts of being a den leader, for me, and it's good that the boys to have more than one leader to rely on.
If you're fine doing den meetings and planning your own outings for your den as needed, all those other trainings are not necessary or even helpful for a den leader. Heck, depending on the quality, most of the leaders may be better off skipping them entirely.
You don't have to do it all, do it perfectly, or do it by yourself. Just do what you can and let others to do the rest of it.
If you need to step back for a while that's okay too. I always look at volunteer opportunities as year-long commitments, then it's time to re-evaluate what I can handle and what I want to do. I would just suggest that you not miss out on something you want to do because you've set the expectations higher than they should be.