What happened with Petunia's powers?

[Julia]

No: HPFGUIDX 137369

While rereading HBP I came across a very interesting quote:

(HBP, UK adult edition, pages 245-246)
"'But she [Merope] could do magic!' said Harry imaptiently. 'She 
could have got food and everything for herself by magic, couldn't 
she?'
'Ah,' said Dumbledore, 'perhaps she could. But it is my belief - I 
am guessing again, but I am sure I am right - that when her husband 
abandoned her, Merope stopped using magic. I do not think that she 
wanted to be a witch any longer. Of course, it is also possible that 
her unrequied love and the attendant despair sapped her of her 
powers; that can happen. In any case, as you are about to see, 
Merope refused to raise her wand even to save her own life.'"

If that could have happened to Merope - the pureblood witch, doesn't 
it seem possible that the same thing might have happen to Petunia, 
the Muggleborn? I mean, if she is not a squib (as JKR said) but 
there is something more to her - can it be possible that some great 
emotional shock 'sapped her of her powers' too? Or perheps Petunia 
always denied her powers and at some point she wasn't able to 
perform them any more...

IMO Petunia's attitude towards magic is quite suspicious, she is one 
of the very few Muggles that are aware of wizarding world but still 
refuse to be a part of it (Vernon and Dudley aren't in the same 
situation - as we can see in OotP, Petunia is REALLY aware of what's 
going on out there, they don't)

Dumbledore says that things like that can happen. I think he knows 
about Petunia and her choice not to be a witch. Of course if it was 
a choice. Maybe she suffered from 'unrequited love' like Merope 
which weakened her powers so much she didn't have any control over 
it anymore?

What's interesting IMO, is the fact that in the 7th book Harry 
becomes 17 and as many of you stated before there might be an attack 
on Privet Drive when Harry attempts to live the house. Will Petunia 
in a face of great danger perform any magic that will save her 
family? Maybe if the great shock and despair can sapped somebody of 
their powers, the opposite can happen if someone face a life 
threatending situation? Perhaps there is some of Lily's courage in 
Petunia? An ability to perform acient magic?

What do you think? 

Julia