I'd guess you are some kind of a social worker and you focus primarily on helping kids. But that's just letting myself fall for the most obvious maximization of your early reversal of the mother-daughter relationship.
My wife Pamela also had a mother who was abusive and didn't want her, but wanted both of her daughters to take care of her instead of vice versa, not unusual, except for the degree of violation and scapegoating she suffered. She became a Jungian analyst after having 10 yrs of analysis herself, beginning at 24 I think. She mortgaged her whole young adulthood to pay for analysis, except that she got a free ride thru Theology school, which she'd hoped to use as a back door to the counseling profession. Her father was the only good mother around, and he died suddenly when she was 11. So she had to marry a guy who was a really good father, but underneath that a better mother too. My mother was also depressed, safe, but not much more than a zombie mother. So Pamela has been a great surrogate mother to thousands of clients. I like being that too (the good mother that both of us didn't have, tho mine was benignly sluggish, which is a lot better than psychopathically hostile). So she likes me to be a good mother to her, which I like doing, BUT IT'S TORTURE SOMEWHAT because she's afraid of mothers (ie hers) as intrusive, devouring, self-focused, hostile and exploitive, so she's pushed me away a lot because her ambivalence about being mothered is so great. And that's been very painful to me. She also doesn't want to mother me AT ALL, or at least that's how it feels to me. So I'm familiar with how it feels to 1. enjoy being good mother (even under the disguise of a father-body), and 2. wish that she'd give me back some of the same quality nurturing. We've talked about all that psychological stuff for much of the 27 years we've been together, so the mutual understanding does get better.
But I sure notice it when someone in my circle of closeness extends a nurturing gesture toward me without me "earning it" first. And naturally I don't get it often, because I somehow seem to be too busy "earning it" to be aware of how those I care for feel about me. And (perhaps like you), almost all of my care-circle have been college students for the last 25 years, so very few of them reciprocate in ways that are obvious enough for me to be struck by them. And men in men's groups, such as I've led and participated in since 1972, are normally so awkward about loving-kindness towards each other (attributable to homophobia, esp since when men dream about warm closeness with other men, there are blatant sexual overtones in the dream imagery). Ya know, I bet my internal pit of "please let me fall into your arms" miSSing mothering is so bottomless that it's unfulfilled no matter what good-nurturing caring seems to come my way. And I really enjoy self-caring, as you do too.
So maybe there's lots of us who've gotten so good at caring for that crying child inside that we don't show our needs outwardly and therefore don't get what we're so fulfilled by giving. (My first great love, however, from 20 to 29, defined her self as sturdy Russian Jewish peasant stock and made herself worthy of my love by mothering me very effectively without my asking while she acted like Girl Friday to my Robinson Crusoe and hitchhiked all over Europe and mountaineered with me. After those 9 years, I had sensed long enough that she was too compliant, even tho I still didn't realize how good she was at quietly mothering me--she thought I was fragile and needed it. So maybe I did get enough, at least for my 20s, and I sure learned how to love nurturantly from how she took care of me. Of course we teach our clients that you can never go back and make up for lost mothering or fathering in your childhood. But apparently that's cold comfort when it comes to accepting our own lot.
Oh, why it's torture for her to love you? I'd say it's because you're giving her the ungrudging nurturance she didn't get, and that reopens her wound. I'd probably get very uncomfortable if I got it all the time, the way I did from Susanne, because after her sexual betrayal destroyed my first sexual bond -- I couldn't fill up that hole -- I didn't want to have another partner like her: follower/caretaker. So I ended up getting neither in all my relationships between that one and my marriage 12 years later, and in my marriage too.
How could she be as loyal to you as you are by your very nature? She never got to "earn love" by being a good caretaker. Sounds like she might have had nothing but grudging caretakers who were in it for the money to make their own (perhaps stunted) lives at least financially viable.
There's really no way around the character structure we've developed in our first 20 years, unless perhaps one uses powerful drugs to whack ourselves out into a drug induced artificial personality, with the sweet, perceptive, tender parts always the first to disappear. No way around, but only thru, the long and hard way, depending on how much and how good the skilled psychological help and self-aware loving we get is. Like "Rock 'o my Soul" where "you got to go in thru the door." There is the loyal-religious way, where the sweet & tender parts can get inflated, and everybody in the congregation is "very loving" because the weekly indoctrination keeps us well aware that God is probably watching us, at least and especially when we're at any church function.
"Nothing in return." Seems to me that it's much less likely you'll get a lot of little signs that you're loved if you don't live together or at least spend a lot of casual, natural time together. Cuz then the only normal way to get those loyal support symbolizing moments is to lean into the digital connection, and that's fraught with the potential for a grudging halfhearted response that hurts more subtly but almost as durably as an outright rejection of your overture. And I bet she doesn't want to make overtures first, because she's MUCH more reactive to "caught off-guard unprepared" grudging responses than you are, since they were probably the norm throughout her fosteryouth.
Oh yeah, you probably already know this: But if your love is a rescue operation of a woman who's way less developed, self-efficacious and self-confident than you are, then you're in a one-up one-down pairing from the beginning. So you can't get back the same kind of love-stuff as you're giving.
I gotta go out into the cold and look for my daughter's likely to be dying Yorkie terrier again, cuz he goes outside to struggle with his blocked elimination functions. So don't stop conversing unless you want to. Birds of a feather flock together.