I'm sorry your first major trip was so crappy! It sounds like it was nothing to do with you but the shop and the assisstant - who sounds awful!
First of all size 12-14 is NORMAL!!!! Actually, its less then average cos if I remember rightly the average dress size in the uk is 16-18, so heaven knows why some shops only stock size 10 in the samples!!
As others have said - dresses come up ridiculously small, and you do have to be careful because some shops will say you need a dress in a much bigger size then you really do so that they can charge you loads for alterations.
I've only been to 2 shops - one shop all sample sizes were a size 12 and I thought to myself - thats not very clever! It must lose them sales because not everyone can tell how it would look on them in their size (I might add that some of the dresses were too small for me and I'm a size 8-10!!!!)
In the other shop the lady had sample sizes ranging from size 6-22 which I though was much more sensible. And when I took my bridesmaids to another shop my sis was worried cos she's a size 16 and the other two are a size 8, but as it turned out all the bridesmaid sample dresses were in a size 16 so my sis looked gorge in them all and the other 2 still did cos the assistant clipped them with bulldog clips at the back!
So I suppose my point it... don't let it get you down. Start afresh, perhaps take somebody else shopping with you and don't feel embarrassed to ask the shop what sizes their samples come in before you go if it makes you feel better. Good luck - you will find the one!!
xx