He Fills Our Gap
"In the same way, the Spirit helps us in our weakness. We do not know what we ought to pray for, but the Spirit himself intercedes for us through wordless groans. And he who searches our hearts knows the mind of the Spirit, because the Spirit intercedes for God’s people in accordance with the will of God." (Rom 8:26-27)
When we don't know exactly what to pray for, or exactly how to ask, or what words to use, or perhaps what God's will is in any given circumstance, God takes care of us. God the Spirit living in us, and God the Father who prayer is being directed to know us, they know each other, and they work it out. God talks to God on our behalf, in accordance with the will of God.
Yes, that's hard to follow, but it happens because God loves us and has given us His Spirit to help us in so many ways, this being just one of them. This certainly must be part of Paul's bold confidence when he tells us "Let us then approach God’s throne of grace with confidence, so that we may receive mercy and find grace to help us in our time of need.".
So sometimes our prayers may be "God, I don't know what to say, but you know my heart", and God is OK with that, in fact, He's got us covered, and the groans of His Spirit will speak exactly what we need to say and exactly what He needs to hear!