5k race fatigue after months of training

Sounds like you're overtraining. I would guess that your easy running isn't actually easy. 30-40mph shouldn't leave you feeling tired if you are training well.
Without trying to sound rude, 21:20 shouldn't really require you to be doing any workouts per say, 45 is not old either so age isn't really a limiting factor here. One thing is are you in good shape, overweight etc? For your workouts, if you want to do workouts, I'd focus on doing longer reps with less recovery, so put the pace down by a bit. You don't need to be hammering 600s and 800s with 2 minutes rest. Really we want you to build up a bit more aerobic base and threshold, so try something like 8 x 1k with 60s recovery, but at what you think your current HMP is, or 5 x 6 minutes with 60s recovery at slightly slower than HMP. It will be more beneficial. You could replace your current workouts with these, and you would be less fatigued whilst building more useful fitness^^

Make sure you're doing a regular longer run too, this needs to be at easy pace and will really help you build up endurance. What pace are your current easy runs also? Might need to slow them down

Well you have been doing the same thing for three months. You need periodization in your training. You really need to do two training cycles per year cause otherwise your just spending to much time in each phase. You should be doing at least one threshold type workout a week and those mile repeats are a little fast. You probably stopped improving around 6wkks with that schedule, stagnated and now your going backwards.

Gotta train under the same conditions you race in (surface, elevation profile, weather, etc). Switch out the treadmill workouts for road/track workouts. Some treadmill running is fine, but try to avoid doing your workouts on them. It's just not the same as outdoor running, and your racing will suffer.

Besides the possibility of overtraining due to lack of recovery in your hectic lifestyle, I can almost assuredly tell you that you were duped into thinking those treadmill paces would translate to the roads.

I posted before getting down to your post, which I agree with completely.

Probably need to cut back to 25-30 mpw of mostly jogging, 8-9 min/mile. Do 8x400 each week at 21:00 pace.

Easy easy: Easy days and long days no faster than 9 minute pace, and many days closer to 10 minutes/mile than 9.

High-quality quality: Alternate 8-12 x 400 (or 90s) at goal 5k pace with 200m (or 90s) very easy walk/jog recovery one week with 5-6 x 1k @ 7:40-:45/mile w/ 200m (or 90s) very easy walk/jog recovery the next week.

Sustained effort: Add a 20-minute tempo run (that can start as 2 x 10 minutes w/ 2 minutes recovery) at about 7:55-8:00/mile (but that pace should really be by feel) once a week.

Appropriate long run: Run 9-10 miles at least once every 14 days.

Also, it sounds like you might be a little overcooked right now, so consider taking a full week with just easy running (nothing more than 60 minutes) and strides, and then starting at the low end of this range. I also agree with the folks who recommended having your training environment mimic your race day environment.

And remember, with a busy schedule, less can be more. And disregard people who say "it shouldn't take much" to achieve your goal. That may be true for them, but everyone is different.

It’s spelled “per se,” and it’s not appropriate in this context.

Some people need workouts to run 21:20, especially at 45 years old. Everybody is not the same as you. It’s difficult for most middle aged people to run 6:50 pace for 21 minutes off of just going out and jogging. Of course, they could hammer runs (or the last mile or two or three) based on feel and that might work, but that’s in direct contradiction with your “easy runs easy” recommendation, and not as effective as doing more structured training.

I agree with a couple of the suggestions that have already been made: get those workouts off of the treadmill and on to real ground, and start incorporating more extended tempo efforts. You can start with just 2-3 miles easy warmup + 2 miles at 7:20-7:30 and go from there (this is assuming your recent 5k was a bit of an anomaly, run on a bad day, which may indicate it’s time for a reset before returning to real training). If you never run at an elevated tempo for more than a mile continuously in training, it naturally follows that the second and third miles of a 5k race will feel difficult.

You are either overtraining or you are overweight. No reason a 45 year old that is putting 30-40 mpw can't run sub 21 5k.

So you need to slow down in your workouts especially your easy days or eat less calories. It's that simple