[Pitchwife]

Originally Posted by Morwen

This mist over his childhood, well the Doriath portion of his childhood, is intriguing and it doesn't seem like normal forgetfulness. Given that the mist lies over part of his childhood are there other things that Turin cannot recall? I also wondered whether one could link this mist to the shadow that is said to have marred Turin's youth.

I think you're on to something here. Immediately before the last quote you give, Beleg asks him,

Originally Posted by UT, Narn i Hîn Húrin

Túrin, have you lived always with your heart and half your mind far away?

It seems he had indeed, and it takes little imagination to guess where his heart and half his mind was - in Dor-lómin, with his mother and unborn sister (and probably his friend Sador and the memory of Lalaith too). I think this was the 'shadow' mentioned in the tale - in the same place, by the way, where the corresponding passage in the 1920s Lay of the Children of Húrin more clearly names

the sundering sorrow that seared his youth

(bolding mine).
But there may be more to it. I'm reminded of another dialogue, where Beleg, when Túrin refuses to accept any 'gifts out of Doriath', tells him:

Originally Posted by UT Narn, Appendix

Then send back your sword and your arms [...] Send back also the teaching and fostering of your youth.

(Sounds like his long patience with Túrin is wearing a bit thin here, doesn't it?)
To me it looks like Túrin, in his grudge against Thingol for exiling him (when he had actually exiled himself), had sub-consciously or half-consciously repressed the memory of any positive aspects of his life in Doriath (such as his childhood friendship with Nellas)which could have contradicted his feeling that he had been wronged (cf his chosen name Neithan) through no fault of his own.