Oh dear. I can certainly identify with THIS:
I find my feelings on parenting one get mixed up with leftover feelings about being one. and
...in an odd way I relax - this attitude I understand!
Yes. In my case, I have a conditioned response that being proud of one's accomplishments and attributing them to skill/practice/effort, and/or being confident is all fine.... as long as it is regarding 'average' ability or accomplishments, that is.
Once it goes beyond that, it goes direcctly into the domain of-- well, being
fat-headed and insufferably rude/arrogant. I guess that in this world-view, the exceptional musicians, atheletes, and chess players are all "someone else." 
Yes, some baggage on that score. No wonder I developed imposter syndrome in a big way and have trouble being assertive. This was my thanks to own mother, by the way.
This was the only way that she had of relating my HG+ behaviors to her own world-view-- to frame this as "not real" aside from in my head, and to coach me to: a) fall short/fail, and b) doubt myself and keep my mouth shut. Otherwise I was anomolous to the point of being "does not compute," so it was easier for her to 'take me down a peg' (sabotage) so that I wouldn't do those things that made it hard for her to accept who/what I was.
I am so fearful of doing that to my daughter, and I just CRINGE when I hear myself talk about her sometimes, because I cannot just make a complimentary observation about her to others. I have this pathological need to add "yeah, that part is terrific, but {this other thing she does is sure annoying/immature/dumb}." Just to make sure that they know she isn't insufferably arrogant and neither am I. 
This is complicated by resentment and, yes, even overt jealousy sometimes. Resentment because my DD's every success is evidence of how spectacularly my own parents failed to actually nurture me or give me what I needed as a gifted child, and sometimes, even toward DD who doesn't "appreciate" all that she has that I didn't. I'm also jealous of her. Maybe 'envious' is the better term, but when she turns into a passive, uninterested slug who doesn't care about any of the extraordinary opportunities she has... well, then I get pretty grumpy over it. She certainly lacks a lot of the drive and motivation that I had... which, yes, makes me even angrier at my own (long-gone) parents.
Being self-aware is a very good start. I know that I've been able to avoid a lot of the subtle sabotage by simply addressing things OVERTLY with my DD.
"Are you afraid that if you win _________, {friends} will feel bad? Be angry with you?"
"It's okay to sometimes do less than your absolute best. There are lots of reasons to do that sometimes-- maybe you're just tired, or you don't want to win that day."
I also ask a lot of questions about the social dynamics of success, to make sure that my DD isn't self-handicapping without being aware of it. This is very important with a HG+ girl, particularly in adolescence. It's all so very complicated for them.
Complicated for us as parents, too. It's amazing to me how seductively 'comfy' those dysfunctional patterns are, when I find myself slipping into that "yeah, but" meme that I learned from my mother.
I've never seen this overtly addressed in any books on GT parenting. The places that do discuss it tend to do so in the context of dysfunctional parenting related to substance abuse/mental illness. Relating to the negative patterns that we learn and not having a template for doing 'better' there, I mean.
