Look at the volumes on www.bitcoinwisdom.com, www.bitcoincharts.com/charts/bitstampUSD, etc. and you might be able to approximately gauge how many BTC could have been bought/sold at that time. Don't torture yourself too hard - for all you know, you could have ended up losing your whole stash in Gox, sold it when it was crashing back to $2 in 2011, or invested it in some scam or Ponzi scheme. Don't be too hard on yourself!