Its a
Cookie
The title was a give-away
For the why part-
I'm an American with a capital A. When I visited the UK, the people there gave me funny looks. But once I changed my name to seven letters long, they became friendly all of a sudden.
Brits call it
Biscuit
Also
I don't have luck on my side, but I have fortune on my side.
Which refers to
Fortune Cookies
Here is more evidence
here's this nifty little gadget used to change me into attractive shapes that I like, but for some reason, if I use it, people criticises me for losing originality and becoming mundane. The gadget is called "my name + six letters long". {****** ******}
That happens to be
Cookie Cutter
The hint that blew the cover
You try to erase me from your memory, and you may feel lighter at heart for a while. But I'll be back every time and start to weigh you down again.
If you have a computer and you are computer-literate, which I suppose you are
You know how clearing Cookies, speeds up things.
Edit, Consolidating all hints
As weird as it may sound, people love the wrinkles in my face.
May refer to
The crusts and choco-chips on the cookie.
Also, Clue 9 and 10 correspond to this, I presume...
(9)Some people think that cookies are used to snoop on other people's information on the web, but they are extremely handy for creating and managing sessions etc.
(10) Grandmothers think we are talking about the cookie referred above i.e. biscuits, but since you are at StackOverflow, the OP considers that you are able to differentiate between a cookie in your jar and one in your browser and temp folder.
Also
Sometimes, women try to avoid me for fear of something – not for fear of me, mind you! But they find it hard to resist me and my charming tanned body.
They try to avoid
Cookies, for the fear of getting fat. but they love it too.
And for this one
I can withstand the scorching desert-like heat – but not for too long! On the other hand, I'm no good in water. Don't expect me to dive into the sea!
This implies that
Cookies may withstand heat but too much heat may melt it's chocolates, while when it comes to water, cookies just become soluble.
And this one
I have physical form, but the other me below is like a ghost.
Which is..
The hints are divided into two parts, 1st for the Cookie i.e. Biscuit aka Wafers, the other is for Cookie aka message given to a Web browser by a Web server.
As for the American with a capital A, I am unable to find any relevance so far. Maybe, I need some Great American Cookie help.