Do you see any difference between the 7 of pentacles and the ace of pentacles then?
Oh, gosh yes! There's certainly the hope of something growing when you plant a seed, but even with earth-suit Pents, the Ace is a very fragile, temporary possibility. Unformed and therefore, if they don't come to anything, easily gotten over. "Oh well, that didn't come to much of anything did it?" Shrug your shoulders and move on.
But with the 7/Pents that seed has not only grown, but it's gotten big enough to bear fruit. You've spent a good long time watering it, fertilizing it, pruning it, keeping it warm through the cold, etc. You've really invested in it. Getting over the fact that there might not be a harvest (or, at least, it might not come when expected)...that's going to be hard!
All the 7's are like that. Like driving on a straight road all night in order to get somewhere then, suddenly, coming up on a roadblock. You're going to be unhappy about that; maybe argue with the police at the roadblock about it (can't they make an excepting and let you thought?). Can you come to terms with taking a detour instead? Maybe even find the benefits in it...even if it's unplanned, and not how you wanted to get where you were going? As with the Chariot card (#7) something has to be figured out, overcome or accepted if you want to drive on.