I think for me it means that we try and think too much / over complicate things. When we try and think too hard about things (I sat there thinking deep thoughts), we find it dificult say what we mean (The words that i wanted were missing). By the time we realise how simple the situation really is, it is often too late.
Its a very moving poem and I shall continue to think about it. If I come up with anything else, I will let you know. I hope your emptiness fills up too.