Thought many times about the fact that, maybe, at times husband and I were so much One that it was hard to say where one begin and the other end. And we both loved it that way. Still, a couple must be One and, of course, we need our spouses.
This is how we feel at the beginning of a relationship, but as psychologists often point out, this is a mistake. Maturity in a relationship somes after this fusion phase, when each person begins to find their individual paths. The role of the relationship changes, and it's important that each partner feels supported in their own projects.
Neediness or dependence in a relationship is not healthy. It means the needy partner has not grown. Sometimes it is inadvertent; in my case, H was always working so hard that I took over all the financial, house and child management. He felt, as he said "like a fly without wings".
It is a pity that to find individual wholeness so much damage and pain has to be caused. On that sense, finding our individual wholeness while we are young and before marriage/long term relashionship is better, is may avoid a lot. But it would had made no sense to break with husband because we were teenagers and maybe, one day, something like this would happen.
It doesn't have to be this way. For many couples it is not. And over dependency is not the only "cause" of MLC. MLC is a deep personal crisis of feeling and identity, which may have had the lid on for so long so that when it emerges, it is like an explosion.
But, as OP says, this is a gift of time to find ourselves. We can use it to grow stronger, to become whole, and find our direction in life. If we fail in this task at this time, we are in danger of becoming bitter, focussing too much on them and how they have hurt us instead of on ourselves and our growth.
It's not easy; this weekend, we discussed that the most difficult thing to overcome is the hurt, resentment and anger. But we know we have to.