Experienced runners still get bad runs... but they have more good runs in the mix, and they're better at understanding why (sleep, fuel, time of day, weather, time of month, recovery, shoes getting worn out, a butterfly fluttering on the other side of the world
)
When I did a 20 mile long run and every step of the way was sore and miserable I knew exactly why... I was poorly recovered from a race and it was 3⁰C grim mizzle (I'd brought it forwards to avoid snow and ice) so mentally it was just one (long) hard run but not a reflection of me as a runner.
This point of C25k is mentally tough. The physical tiredness is kicking in. You don't fully feel/ appreciate the progress you've already made yet. You can see another 5 weeks of the plan stretching ahead...
But it is a plan. The plan works. Hundreds of thousands of people have followed it and become runners by following it (even me!!!). Physically it's tiring because your body is adjusting to the changes and improvements in your body (CV system tends to improve quicker than muscles) Mentally it's hard because you're doing something new and not taking an easy option.
A lot of people find a couple of miles hard because it takes that long for their body to warm up and relax into it and running gets easier beyond that distance. Another thing that's hard about being a new runner is pacing. Your PE teacher was wrong, slow is the way to go. Take it easy. Slow leads to stamina. Speed comes later when your body knows what it's doing. It's not needed for C25k.
Over a decade after my first C25k and numerous HMs and a marathon later, I still use C25k and Nike Run podcasts to talk me through runs when my brain isn't playing ball. I also go to parkrun to push myself into harder runs; so much easier to run with a flow of people than push yourself to do it! (Parkrun is a good beginner friendly option, lots of walkers & run/ walkers at 35, 40, 45, 50+ minutes)
Hang on in there. You have made progress already. The plan is working. 