Hi Mic, I've been struggling bad with scrupulosity for about 2 years now. The three years before scrupulosity started I've had obsessions about the existence of God, but it ended and scrupulosity began.
I know your pain... I am right there with you.
You've described it well... it is impossible for us to understand and reassure ourselves with grace.
What I've started doing is taking breaks of trying to reassure myself. Basically, once the fear and anxiety comes that I cannot understand grace, I stop trying to reassure myself and go do something else and wait for the anxiety to calm down. It takes hours or days for my brain to calm down. Then I think about grace again and it triggers the anxiety, so again I stop trying to reassure myself and keep myself busy with something else in order to calm the anxiety down. This way I am trying to desensitize myself to the anxiety. This is the only way there is to fight OCD. It can take a long time to desensitize yourself. It took me many months to desensitize myself to the OCD of God's existence... the mention of the word "God" would create fear and anxiety in me that I dunno if God exists which means I'll go to hell.
I've sure you've realized, that when you get anxiety about salvation and try to reassure yourself with grace, it never works, or if it works, it works for a few seconds or minutes and then disappears again. Not only that, but the more you try to reassure yourself, the worse the anxiety and doubt grows. OCD cannot be fought with reassurance... only with desensitization. It is tough to force yourself not to reassure yourself... but you gotta practice and get used to it. Once the anxiety appears, stop reassuring yourself, as tempting as it might be, and go keep yourself busy with sometime easy and fun and relaxing to try to distract your thoughts from the fear of hell. The anxiety and fear might be strong (perhaps the fear of going to hell), but don't try to reassure it with Bible verses or anything else. Just tell it to stay there all it wants, and that no matter what, you won't try to reassure yourself. Encourage the anxiety to come and give you its best shot, and that you won't fight it at all. With time, you will desensitize yourself, and you'll be able to think about grace and salvation with no anxiety being triggered.
I am in the process of doing this... so far my anxiety has already decreased in the week that I've been doing it. It takes time, but it is the only thing that works.