I think when you lose your sense of self, when everything you do is for the other person without consideration of your own needs or wants, you're in too deep. That's how I would define being too much in love--or co-dependent.
My husband and I get made fun of, even by his mother, because we still act and feel like honeymooners. Yet, even though we spend a lot of time together and have a lot of shared interests, we each have a strong sense of self. I love him with 99% of my heart, but my relationship with him doesn't define who I am. I keep that one percent for me, and that makes me more interesting to him in the long run because I'm not totally dependent on him for my happiness. Does that make sense?