How do I help my pup overcome a fear of large dogs?

I have a 4 and a half month old corgi pup and yesterday I took her to the dog park for the first time. My boyfriend and I practiced come and off leash walking. Now she leaves with a large Labrador and my bf's parents have an over sized border collie with both she is fine with and i thought she would be fine around other dogs but when two shepherds (German and a large mix) approached her yesterday she flipped out and started yelping. I knelt down and sat her on my knee and their level and patted them to showed her that they weren't gunna hurt her but as soon as I walked off she was still terrified. How to I help her with this? I don't want her running away from bigger dogs her whole life!