Why are you trying to ride so fast?
The minimum speed you have to maintain to finish in the time limit is 15km/h. Yet you have expectations of finishing a 300 at 12-1/2 hours, which is an average of 24km/h. You are "new" to this and you are trying to do this?
Randonneuring is a challenge for you to finish, not beat the other guy. Times and averages are pointless because the winners are those who finish under the cut-off times.
Slow down. At least for now. There are several benefits. First of all, you can pay much better attention to your rehydration and refuelling routines. They need to be developed to being a habit.
Secondly, you will use your glycogen stores in both your liver and muscles at a steadier rate, and combined with a better rehydration and refuelling routine, you will be far less likely to bonk.
This is unusual advice to be given by me, but even think about turning your bike computer 180 degrees on the handlebar do it's facing downwards and you cannot see the speed you are doing.
That way, you can ride at a comfortable pace for how you physically feel, rather than be ruled by numbers of the face of a tiny computer screen. I think that you should be able to maintain the 15km/h minimum pace without any trouble, and you can reference what you are actually maintaining at the checkpoints.
And yeah, the reason why other people do so well is because they indeed are prepared to put in two to three times the distance in training on the bike. Up your training a bit, at first to develop your routines and stamina. You can incorporate pace work later to help improve your event speed.
Right now, I would suggest you take a month's break from riding altogether. The closeness of each recent ride means your emotional woes are just compounding. And when you do get back, do a couple of 200s first to get your rehydration and refuelling routines right first, before graduating on to the 300s, 400s and 600s.