Heya, Edward, welcome back! I'm really glad to hear that you have been making progress. In my experience, setbacks like that are part of life and reaching out helps me get by. I bet the same will be true of you, so welcome!
I've had some pretty dark times in my life, too. I remember taking a walk about 6 years ago and being afraid that if I didn't ever feel better, I might think about suicide. It was related to a broken up relationship as well. I thought I'd share that so you don't feel alone, but I do think that feelings of that sort are often a wakeup call that a doctor or therapist could really help. Of course, our community doesn't have any formal training regarding suicide, so we can't assess what sort of help would be best for you. I'd suggest calling 1-800-273-8255 or Googling the phrase "suicide prevention" to find the resources that are best for you. It's better being safe than sorry.
In terms of the SEP, if you want to do it, I'd suggest starting on day 1. However, I'm not sure it is the right thing for you right now. As a completely untrained amateur, my hunch is that you might be suffering from depression. You can Google "depression screening" and take some of the free depression screening tests, but they can't replace a consultation with a licensed professional. This is relevant to the program because Day 4 of the SEP says the following: "This program is intended to help people explore feelings that may be causing their TMS/PPD. If you are depressed, you should consider whether you are ready for this level of exploration, and revisit the structured educational program once you are ready."
Why would that be true? Well, I know that when I feel bad, I tend to think about the things that make me feel bad - which makes me feel even worse. From a scientific perspective, one of the purposes of negative emotions is to focus our attention on bad things (like a lion approaching) so that we can do something about them. Unfortunately, this can lead us to focus too much on the negative if we do journaling when we are down. I know this because I used to do it all the time when I was feeling down. It only made me feel worse. Hence, doing journaling in the SEP when you are going through a rough spot may not be the best idea.
My personal thought about how to get through this rough patch? Throw yourself into something you love. Your mind will want to ruminate on the past, and the best medicine for that is to immerse yourself in something else that makes you feel good. If you find that you can't take pleasure in the things that you usually take pleasure in, that may be a symptom of depression and another clue that it may be time to get help. Hopefully that won't be the case, though, you will get a good start on the road to recovery.