There are a couple of key things, one of which you have highlighted, but for me over the years it has come down to two things:

We panic when in the lead in the last few minutes.

The players lack the confidence to either close out the game, or comeback and win it.

Scotland vs. Italy 2008- 20-23 last minute drop by Italy secures win

Scotland vs. Argentina 2008- 15-21 after leading 15-10 well into the second half

Scotland vs. South Africa 2008- 10-14 after leading 10-0 at half time, Godman misses penalty from right in front of the posts.

Scotland vs. Argentina 2009- 6-9 last few minutes drop goal, dominated match

Scotland vs. Wales 2010- 24-31 after leading by two scores in 70th minute

Scotland vs. Argentina 2011- 13-12 after leading 12-6 72nd minute

Scotland vs. England 2011- 16-12 after leading 12-9 75th minute

All these were games we were well in top in, should have but ultimately lost in those last 10 minutes or so going back to 2008. Games which our immediate 'comparables', Ireland and Wales, most likely wouldn't have lost.

Meh, I think it is just more hurtful to say the English deserve credit than to say we are sh*te