I always took this as saying that the wrong plans won't prevail--God knows best. But after experiencing extreme disillusionment, and realization that I once desired something I cannot have, this gives me hope that God saw this coming and is moving me past this into something better--something worthwhile.
#19 "Purpose tells us this is not a whim, not a passing notion. God isn’t capricious. Everything He does is according to His will." http://www.lizcurtishiggs.com/2014/09/your-50-favorite-proverbs-19-best-laid-plans/
“Listen to advice and accept instruction, that you may gain wisdom in the future. Many are the plans in the mind of a man, but it is the purpose of the Lord that will stand.” Proverbs 19:20-21 ESV http://bible.com/59/pro.19.20-21.esv
Jesus has a funny way of messing up all of your plans to allow room for His to take place. I imagine Him chuckling and saying to me, "These ideas of yours are dreadful. Now let me show you all the beautiful things I have in store for you."