I agree with Rowan ...
Slow down!
When I first started reading, I thought you were DNFing because you were too slow to make it, and then I see you're attempting to do these rides as though you were riding the local races, and that you've got heaps of time to finish.
On that 300 where you were on pace to make it in 12:30, you could have probably walked that last 8 km and made it in time. In fact, from the 275 km point where you first started feeling bad, you had approx 8 hours to cover 25 km ... you could have walked that and finished it with a couple hours to spare.
The time limits for the SR series are:
200K - 13.5 hours
300K - 20 hours
400K - 27 hours
600K - 40 hours
Just relax. Take it easy out there. You don't win any prizes for finishing in some record time ... you get the same "prize" as I do, when I finish a 300K in 18.5 hours.
I went through a similar set of DNFs recently too. I had several really good years of randonneuring, then developed a rather serious case of DVT which knocked me right back to struggling with 6 km rides. Yes ... six. Not sixty or six hundred. When I started randonneuring again, I struggled and DNFd many events ... even now, 3 years later, the longest I can manage is 300K. But here's the thing ... I'm slow. I'm too slow to finish the events in time. I'm determined and motivated to finish ... but I'm just not fast enough. I'd like to be in your position ... fast enough to have about 8 hours to play with as I'm wrapping up a 300K!
If I had been in your position, feeling a little bit bad at the 275 km point and with approx 8 hours to finish the last 25 km, I would have found a nice place to stop and take a break. I would have pulled into a McDonalds for some fries and an ice cream. I would have stopped at a park to get some cool water, to eat the cookies I brought with me, and to sit under the trees for a little while. I might have even taken a short nap in the ditch.
When you start to experience the fear and disappointment, knowing that you're never going to finish in time ... get back to us about "sucking at this sport".
Meanwhile, either slow down and finish the rides at a pace your body can handle ... or go back to racing.