As Good As You Are Right Now
You're only ever as good as you are right now. Over the course of years of running, there are microcycles and macrocycles. If you draw a line from the first day you started running to today, that line will almost certainly move up and to the right. On the whole, you're always getting better in some respect, or at least better relative to what your total genetic capacity is for your age. There are microcycles of getting worse or setbacks, just as there are microcycles of "peaking" or supercompensation. It can be hard to look back 1 month, after recovering from a minor surgery and a case of sniffles, and see that you haven't gotten any better. Maybe by the numbers you even got a little worse, but you have to look backwards and forwards further than that. The road is filled with rolling hills, but most lifelong runners will see that they're always ascending in some way.
Richard Schick is in his 70's. He runs on average 50+ miles a week, and still gets out nearly every month for 50k race efforts. Whenever I talk to Schick, I get a new tip, or something that he's testing out. He's been doing this for over 40 years and is still testing out new methods, new ways to eek out a little more from his body. Are his race times anywhere close to where they were in his 40's? No, of course not, but I'd venture that he's a better runner now than he was then because he can still do it at all. When you're young you can get away with a lot of stupid mistakes, things you thought were making you better but were really holding you back. Youthful vitality masks many errors. Hundreds of tweaks and thousands of hours of fiddlefarting around with his training have kept him going after all these years, when men decades younger have hung up their shoes and retired to a quiet life of trout fishing or walking their dogs. Schick's age has gone up and to the right, but so has his skill at running.
The perspective is that you're only as good as you are right now. When you toe the line on race day it doesn't matter much how much training you lost over the last month because of that cold you had, or that late night at work. You're going to have to shrug and accept, even enjoy where you are. This race is a blip on the timeline. One battle in a lifetime of battles. If your perspective is that everything you've done before leads up to this, you will almost certainly be disappointed. If your perspective is that this is the last shot you'll have at testing your training, your future self will be disappointed.
Accept that you’re as good as you are today. Tomorrow you’ll be as good as you are tomorrow. And the next day. Feel the continuum and not the discreet measurement of a race or a point in time measurement of your ability.