Hi GreenGlit and congratulations on your 27th bithday
I've been a successful kid and am graduating from medical school in a few months.
This is awesome! A huge accomplishment

I just got married exactly one month ago to a wonderful and supportive man. I have a happy and peaceful life with him.
Also nice to hear married life has been treating you so well
But this is my first birthday since things with mother have gone stale.
... .
But really the message she sends is that she has stopped loving me.
... .
She sent me a simple text wishing me happy bday, and a check for a gift, and it just feels so - forced. Fake. Like motions she has to go through as my mom, but not because she loves me.
I don't know how to deal with that.
I can understand how you would find this difficult, especially on a day like today, your birthday. You mention not knowing how to deal with it. I think this in a way might actually also be true for your mother. As you say she is sick, and given the reality of BPD I think it's fair to say your mother has very poor coping skills. As hard and unpleasant as it can be to accept, her current behavior might actually be the best or only way she knows how to deal with the LC. In her disordered mind it might even make perfect sense.
As dysfunctional as your mother's behavior might be, this however doesn't necessarily have to mean that she doesn't love you. Perhaps she just doesn't know how to express her love.
Since she has BPD, it is also likely she has a very shaky sense of self and severe difficulty with not only regulating but also understanding her own emotions and those of others. Really understanding how her actions affect others is something she might struggle with. Perhaps it's not so much that she doesn't love you, but that she doesn't understand love.
Having said all of this, I realize that on a rational level it is way easier to accept these things than on an emotional level. Deep inside nearly all of us probably still and might always long for the fantasy parent we never had.
Take care and I hope that in spite of these difficult thoughts and feelings, you will still be able to enjoy your birthday with your husband