A repeatable training routine leads to consistent training, leads to consistent progression and ultimately consistent performance. Consistency begets consistency. The great Steve Moneghetti makes an analogy with a phone book (remember them??), where a single session is like a page that on its own lacks strength. But the more consistent sessions you do, and more pages you put into the phone book, the stronger it becomes, until it is like a tome that is nearly impossible to tear and damage.
When you consider training in this context, then the prospect of missing a session...or just having a bad session...is just a blip on the radar; a drop in the ocean. Just move on. If it was a bad session, understand why and rectify any contributing factors...and then move on. In (almost) all cases, don't try to make up for it (however, there may be some exceptions - talk about it with a coach). It's impractical to think you can train well all the time. Just move on, to add more pages to your phone book.
Even if your training background is more like a brochure than a phone book, the same thing applies for missed or bad sessions - just move on. Similarly, if you have a bad race (remember those??), perhaps give yourself 24 hours to stew on it, then move on, for your sake and those around you. Training - and racing - should be something that builds you up both physically and mentally, so it's not worth dwelling on a single session. Just try not to make it a pattern of missed or bad sessions.