On a practical level, if you haven't, check out www.cfidsselfhelp.org. It's a website by a psychologist who has personal experience with CFS and has free advice about pacing. One of the things they talk about is tracking your symptoms, activities, events, etc. closely for a period of time and seeing if you can find patterns. Some of CFS is totally unpredictable but others might be somewhat predictable. Just remember that one of the stranger things about CFS is you could feel reasonable doing something but that the effects from the activity can hit hours or even a day or two later. That's what post-exertional malaise is. For example, I know when I get a sore throat without other cold symptoms, that it's a warning sign, I better start to rest more or I'll get more symptoms, have a longer exacerbation down the line. I have an idea of how much I might be able to do before getting even sicker. In terms of the tachycardia, one consideration would be to look up stuff about orthostatic intolerance/ POTS, get tested for it, and get treated. Especially if you get the tachycardia and anxiety with longer sitting/ standing periods. People with these diagnoses can get the feelings of anxiety without having anything psychologically prompting it but over time, I would guess the biological feelings can make one more anxious psychologically. Early in my illness, I had what felt like 2 panic attacks coming out of nowhere for absolutely no reasons (I am not an anxious person at all). My ME/CFS specialist explained that that could have been the OI.