I don't think a person with PTSD can 'forget the past' any more than a person with a physical disability affecting their mobility can 'get up and walk'.

People assume that as they have never had to deal with PTSD that they can judge how quickly someone should 'get over it and move on'. They don't realise it doesn't work like that.

How I wish that I didn't have a malfunctioning brain which gives me different ways to experience of the worst parts of my life without warning and completely against my will!

I hope you manage to either explain to them what PTSD is, or manage to, at least to some degree, ignore them and know their ignorance guides them and in some small way it is good that they don't get it, because they don't live it.