There are two types of tefillah. One is the tefillah established by Anshei Knesses HaGedolah – the Shemonah Esrei. The other form of tefillah is the tefillos that one says in his own words.
It states (Bereishis 48:22) ובקשתי בחרבי, "with my sword and with my bow" (Yaakov Avinu conquered Shechem with his sword and bow). Unkelos translates these words as ובבעותי בצלותי, "with my tefillah and with my requests."
"Tefillah" (בחרבי, sword) represents Shemonah Esrei, and the "requests" (בקשתי, arrow) represent the tefillos that one says in his own words.
The Meshech Chachmah (and also the Brisker Rav zt'l) explains that a sword has the potential to kill because its blades are very sharp and its tip is pointy. An arrow doesn't have the power to kill on its own, but when one pulls back the arrow in its bow with strength, it flies out of the bow; it becomes powerful and can cause damage.
This is the difference between תפילה, the tefillos written in the siddur, and בקשה, the tefillos one says in his own words. When one says the nusach of the tefillah of Shemonah Esrei, the words themselves accomplish so much and have the potential to annul all decrees. (See at length Nefesh HaChaim, Shaar 2, ch.11 and onwards). The words were established by Chazal with deep kavanos and ruach hakodesh, and therefore, the tefillah accomplishes so much, even when one's kavanah is absent or minimal. Whereas a בקשה is a tefillah in one's own words. It doesn't have this intrinsic strength, but since the tefillah comes from the depths of his heart, it is like an arrow pulled far back. It has the power to pierce the heavens.
Therefore, a person can use both forms of tefillah and daven at all times.
