“Excuse me? After all I’ve done for you and all you’ve put me through?”, you think. Next comes: “Wait…what does that mean, anyway?”
It could mean a lot of things.
Let’s take a poll as to what it means when your partner has a mental illness:
A. Exactly what they said: they still care about you, but the romantic spark of being “in love” is gone.
B. They never loved you in the first place, but are just realizing or admitting to it now.
C. They are struggling so much with their mental illness that they are incapable of feeling emotions of any kind, including love. Therefore, “feeling no emotion” = “I must not love you any more.”
D. Your partner is experiencing a major life change with having a mental illness, and is re-evaluating their life, which includes your relationship.
E. This is another version of “It’s not you, it’s me” when someone is looking for an excuse to end a relationship.
We could argue that any and all of the above answers apply, and I would agree with you, but the best answer is C.