There is no such thing as loving someone "too much".
A mother willing to die in place of her child. Would we call that loving too much?
A man willing to risk his life to protect his family from an intruder.
Would we call this loving them "too much"?
It's only when we discuss (romantic love) that we buy into the concept that it's possible to "love too much' and yet most of these people would not die for the people they "love too much".
Usually when one believes they love someone "too much" what they're really saying is (they don't love themselves enough!) In one way or another they have given up being themselves or sacraficed their wants, dreams, and needs in order to fit into the other person's world and now they realize their mate would have never done the same for them. It's not so much about "loving too much" as it is the feeling of "inequality of the love" in the relationship.
Having a lack of confidence and low self-esteem will lead one to feel undeserving or lucky to have someone in their life and therefore they walk on egg shells or live in fear of losing them. Once again I would say this is NOT loving someone "too much" but rather Not Loving YOURSELF Enough!