Once there was a girl in the marketplace selling apples.
Some thieves came and started stealing apples.
She just sat and gaped at them.
Someone came over and said, "Don't just stare, Stop them!"
She said "Why should I? They're just continuing and doing it anyway."
"Don't be foolish" she was told "Save whatever you can!"
The same is when we are trying to daven and we didn't manage to have Kavanah for some parts of the Tefillah. We shouldn't give up; we should instead at least save whatever we can!
If you are in middle of davening and realized that you already said so much of it without any thought, at least grab on to whatever you still have-whatever is still left of your tefillah.
If you are in middle of bentching and spaced out a little, continue wherever you are up to with some extra thought and concentration! It's never too late-as long as you are still in middle you can have kavannah and think about the words you still have left to say!
The same is with anything we are working on, even if we fall a little, the key is to not despair, but rather to still hold on to whatever we do have and build on it more and more!