Let me count, average, and fit a least squares curve to the ways
Can you love God too much?
Of course not. "Too much" implies "too little" and "just right;" in other words, a measure, which in turn generates a suitably measured response. We are not to love God in a measured way; we are to love Him with all our heart, and all our mind, and all our soul, and all our strength. Our loving God too much is like a cup holding too much water; as much love as our hearts can hold is how much love they ought to hold. (Though you usually don't want water to slosh out of a cup, while you do want love for God to slosh out of your heart, as it were.)
But you can do too much as a means of loving God. Put another way, there are disproportionate means of loving God.
When we say "proportionate," we often mean something like "on the same scale as" or "about the same size as." We might say that reading thirty books is proportionate to reading thirty-four books; the time and effort involved are roughly the same.
"Proportionate means," though, aren't means that are "on the same scale as the end." If they were, then, since our love for God should be unmeasured, so should the means by which we love Him.
Instead, proportionate means are those means proportionate to achieving the end. If my end is a fine rum swizzle, the proportionate means are combining one part sour, one part sweet, three parts strong, and four parts weak. No matter how much I want a rum swizzle, filling my glass with rum won't produce one. All parts rum is a disproportionate means to a rum swizzle.
(If you prefer a more traditional example, you want all the health you can get (as an end), but that doesn't mean you want all the medicien you can get (as a means).)
To act in order to accomplish some end is necessarily an act of reason. If we want to achieve something out of our love for God, then what we do to achieve it is determined by both our love for God and our reason. And if what we do is determined by love and reason, we can (although we usually wouldn't) say that it is "measured" by them, that there is a "too much" and a "too little" and a "just right."
For the interior act of charity has the character of an end, since man's ultimate good consists in his soul cleaving to God... whereas the exterior acts are as means to the end, and so have to be measured both according to charity and according to reason.
The good news in all this is that, if you find yourself asking, "Am I doing too much for God and for my neighbor?," the answer may well be, "Yes."
The bad news is that the answer depends on whether what you're doing is proportionate to the end of loving God with your whole being, and through Him your neighbor, and not on whether it's proportionate to the end of loving them as much as you happen to today.