Every time I read Psalm 91 I get even more confused about it. Could you please explain what the meaning really is?
The simple meaning of Psalm 91 is that we should trust God and he will protect us from harm. (Keep in mind that what God considers harm is on a spiritual and eternal level, although he will also work the physical to our good.)
When Satan tempted Jesus he did quote verse 11-12 (quoted in Matthew 4:6). Clearly he was taking the obvious meaning I mentioned, although he may have applied it specifically to the Messiah.
When reading the Psalms one has to be careful not to overanalyze them. Generally, read the whole thing as if there were no verse divisions. That should give the overall sense. One of my objections to studying poetry in English classes was that the teacher tended to expect us to get more out of a poem than the poet clearly expected. It is the same with these poems. A poem is effective as a whole, rather than a sum of many parts.