My first year of randonneuring went pretty well. I didn't finish with the top guys every ride, but I had a comfortable time cushion at every distance. My second year of randonneuring didn't go nearly as well. I had some health issues and got sick all the time in the winter and left me overweight and in poor shape for the SR series. I finished dead last at every distance in the series, including finishing the 200 just as time expired. I think I had something like 2 hours total under the time limit in the 4 rides. Rather than taking it as something to be ashamed of, this experience gave me a lot of confidence. I decided to start with the basics and correct my mistakes one by one. The 600 I just finished went really well. I don't think I ever got behind in my eating. I got enough sleep.
In the case of the OP, it sounds to me that the issue really boils down to eating. Figure out why you aren't eating and correct that. I get into this mode where I need to eat, but the controle is only 8 miles away so I'll just eat there. That's a fools game and I've stopped doing that. It's amazing how long it can take to ride 8 miles when you really need to eat. I also was too reluctant to sit up and fish something out of a jersey pocket to eat. So I got a food bag that I put on my stem. It has really helped me, and I don't care if it makes me look like a dork.