The Sefer HaChinuch writes that through the Mitzvah of Tefillah, Hashem “made Klal Yisroel know and opened an opening for them such that they attain all of their requests for the good – and that is that they request them from Him, blessed be He – since the wherewithal and ability to provide for all they lack is in His hand; as He will have the heavens answer ‘to all that call Him in truth,’ …and that there is no prevention and impediment in front of Him for anything He desires.”
Knowing the above, where is there room for worry in the heart of any Jew?! The Creator of the entire universe commanded you to ask of Him all your desires so that you may attain them for, “there is no prevention and impediment in front of Him for anything He desires.” However, one may ask, is there indeed no impediment before His granting our every desire? How about our many sins? As Chazal say (Sifri, brought in Rashi Devarim 32:18): Many times when Hashem is about to bestow good upon us we “provoked Him to anger and, as it were, weakened (תש) His power so that He could not do us good.” After all, there are certain rules Hashem set down by which he runs His world and one of them is that our sins can prevent Him from bestowing good upon us.
The Sefer HaMiddos reveals to us an awesome benefit of Tefillah with whose knowledge this question falls away. “Great is the power of Tefillah, that anytime one prays with concentration to his Father in heaven, the Holy One blessed be His name accepts his prayer, furthermore He also forgives all his sins, as the Pasuk (Tehillim 5:5) says: ‘For You are not a G-d that seeks wickedness.’ And the Sages said, ‘The Holy One, blessed be He, does not desire to punish His creatures, rather He desires that they pray unto Him so that he might accept them. And so too, the Scripture (Daniel 9:19) states, ‘O G-d, hear; O G-d forgive.’ Now that we have no prophet, no priest, no sacrifice, no Temple, no altar, who will atone for us? Even though the Temple is no more, prayer remains available to us.’ Therefore, Daniel said, ‘Hashem listen, Hashem forgive!’ And the Sages said, (Yalkut Shmuel 87): ‘Prayer is even greater than the sacrifices of the Beis HaMikdash.’”
So while it is true that when we “provoke Him to anger He, as it were, is weakened so that He could not do us good” (Sifri above) that is irrelevant in the face of heartfelt prayer in whose presence we are promised Hashem forgives all our sins!