Worrying about past success is as bad as worrying about past failures. They can generate an anxiety from the expectation to match or outperform oneself—which can be crippling. An epidemic in the creative fields, but an experience that can affect anyone.
With time—as you mature, age, develop skills and have more experiences—you may look back on your former self, or work you did in the past, with embarrassment, difficulty, and maybe even regret; in most cases, this is good, because it means that you have grown, that you have improved, and that you will most likely never make that mistake again. On the other hand, you may look back on it as a burden, which can be crippling; this tends to happen if the phase of your life in question was a particularly blissful or very creative time, and you are worrying about your ability to top it, match it, or worrying that you haven’t improved and maybe got worse.
This is a very common experience amongst creative artists and top performers—they feel compelled to match or ‘out-do’ themselves, or their own work, which can be daunting, and even immobilising. The problem with this is twofold: firstly, worrying about the past like this is foolish; it is done, cannot be changed, and is a sorry waste of time, energy and morale; secondly, as long as the worrying continues, the produced work going forwards will very likely be sub-par—because it will be produced with the wrong intention.
Whether the comparison you make is to your former self—your beliefs, ideas, energy, passion, skills, ability, etc—or to a project you completed, at that time, so you were very likely not thinking the same way you are now (worrying about the past), but about that very moment.
The problem is one of intentions; if you try to replicate this piece of work with the sole intention of trying to replicate it, if it was something special, then you will most likely fail, and, have a very miserable time in the process. If you instead focus on the task at hand, on your art, on the moment, then you relieve yourself of a harmful burden, and, granted that you have grown as a person since, you will probably produce even better work—but you can’t know that NOW, and you shouldn’t try to…