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Alright, so, with more background acquired I'm ready to try and propose a solution. Hope it helps.
You got right to what I was thinking when you mentioned that your relationship is more parent/child than two adults. Cross clan relationships between grownups and screwups always feel a bit skeevy, because can he/she really consent when the alternative is to starve? Its much more palatable to think of the whole thing as a strange version of parent/child.
If we do that, your dilemma is one that has been shared across the ages. How do I make my kid clean his room? Middle ages, folks would say to grab the belt. Recently, folks would say you need to negotiate, but screwup clan wouldn't be screwup clan if they could trust their future selves to follow through on current commitments.
I'd recommend that you basically kink the feeding tube. Somehow, the situation has come about where you are paying her to go to college. That's leverage. Give her carrot/stick of if she goes along with you you'll pay for her psychiatry, if she balks you'll stop paying for her college. It's such a massive hassle to rearrange her entire life that she'll probably take this deal. (If paying for her psychiatry isn't in your budget then this whole thing is a non starter, since its not like she's going to be able to pay for it.)
A word of caution: In a relationship between grownup and screwup it is nearly always the grownup who gives way whenever there is a dispute. You'd expect the opposite, but I've seen it over and over. The general reason is that the powerless party would have to recognize the truth of the situation if they ever backed down, while the person in the stronger situation can magnanimously give way.
Example: I want to go to bridge club. My kid wants to go to karate class. If I back down, I sleep just fine that night. I made the choice to give in and go to karate instead of bridge. If I don't, they have to look at their life and face the fact that they just got dictated to. It's sobering.
Ergo, the near constant result of grownup clan putting its foot down is some kind of self hostage taking. "You know I have a temper", etc. Remain steadfast, because if you give in this time it'll just teach them that you'll give in the next time.
I'm not paying her tuition; her brother is (with financial aid). Most of the money I give goes to food. Also, I'm living at her house and sleeping in her bed, and I really don't want her to make me go back to my parents' house where my wheelchair-bound mother will drive me crazy. That gives her leverage, too. (And I'm barely not a screw-up myself...)