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The quote is from chapter 1 of Emma: "The real evils, indeed, of Emma's situation were the power of having rather too much her own way, and a disposition to think a little too well of herself; these were the disadvantages which threatened alloy to her many enjoyments. The danger, however, was at present so unperceived, that they did not by any means rank as misfortunes with her."

One of the definitions of alloy is "the state of impairing the quality or reducing the value of something."

Emma was used to having her own way, and she thought a bit too highly of herself and her abilities. She didn't recognize these - at first - as disadvantages. However, when she begins to realize that she doesn't have all the answers, she doesn't even know her own heart, and by trying to arrange the lives of others in what she thinks is the best way (example-Harriet) she causes herself problems she never imagined and, thus, finds that she can't enjoy her many blessings to the full.

In other words, these two things (having her way & thinking her way is always right) are, in the end, what causes her problems and gives her moments of unhappiness and pain. Or, you could say these things interrupt or damage or lessen or mar or spoil her happiness.

Does that make sense? (I'm not sure how well I put it.)

an alloy is a combination of metals.

so i guess she was saying that the character was high strung and had a lot of confidence, to the point of a big ego.

except that these were disadvantages which disrupted her normal life and her leisure activities.

It means that because of her personality of thinking highly of herself, her enjoyments are debased because of adding this inferior quality in.

to limit. Since the girl is so proud and stuck up, she probably will not have as much fun as others.

This is from Pride and Prejudice isn't it?