*appologies for the length of this post*
Salt..what you've said about her sounds *incredibly* familiar. So..I could be projecting. However, what did it for me was...well frankly..coming here. My guess is she's been told her entire life that she was never good enough. When she did something, it was either not good enough or barely tolerable. For a person who's idealistic and perfectionistic at heart..that's devastating.
I personally did not have a frame of reference when I left home. Meaning...I constantly was on edge that what I did would be received well with people or not. And I automatically assumed I got lucky if they happened to love it. If they hated it, or didn't understand my actions, I automatically took that as me being a failure. It also means that you *stop* doing things...coz then at least you cannot do anything wrong! And if you withdraw and don't do stuff and stay in the background, then at least you're no burden to anyone but yourself.
The reason she flourishes when she has a boyfriend is becoz someone needs her. Someone appreciates what she has to give, sees something in her that she for the life of her couldn't find. She sees all of her flaws, none of her talents. For that matter, those things that come natural to her probably are oblivious to her in that she feels *anyone* could do them, so they're not special. She probably doesn't even realize that she is a natural at them and that others can in fact struggle at those points. And having someone love you, despite all those flaws, without judgement tends to really soothe the pain. But it doesn't take away the cause. He may see you as special, and you do wanna feel that way, but at the same time you feel like a fraud..coz the only way he could possibly see you that way is if you somehow deceived him, as he's not seeing you for who you really are. Then again, if he did, he would most likely leave me..which you so didn't want. So you feel guilty about deceiving him, but need him to feel better about yourself, which causes a new set of guiltfeelings. See how that loop forms?
The first thing I did was disable the guilt. Not easy. It's hard to believe that you're only human and allowed to make mistakes when you've been told all your life that that is the one thing you should avoid. But she needs to hear it, over and over again. And she needs to see that she too is tolerant and ok with others making mistakes and that those other people pick themselves up and go about their business without feeling guilty about making mistakes, or at least dont' keep punishing themselves for it.
Second, I decided I needed to know what I was good at. That's when I discovered MBTI. I took a bunch of online personality tests to see what I was like. And why *being the way I was* was good. How it did have benefits. How other people weren't always right about what would be good for me. How I wasn't a failure necessarily, but I had strenghts to compensate for those weaknesses I constantly got called on. And how I was too scared to use them to do so, afraid that I would suck at that too. That's where this forum came in handy. You get to see how different personalities (which you have a sort of rough draft of through MBTI, a structure to grab on to and work from) interact with each other, how they react to one another, what their motivations are, what mine are. And then it dawned on me that they didn't know everything or weren't always right and I was always wrong ( I know, right?). It was *just* their way of being. And mine wasn't flawed, it was equally valid. That realization, however stupid it may sound, took me till last year to fully grasp. Before, I knew it..I just didn't *feel* it.
Last, I started to gain new insight into how people work, what their motivations are and how that could benefit me and how it could make me happy, change my point of view, in other words, further my personal growth in a way that Fi did approve of.
I'm pretty sure, much of this applies/will apply to your friend to some extent, at least, that's how it sounds from what you've said here.
Tell her from me that she needs to do this, and take the time to work on herself. Right now, she probably doesn't feel like she's worth it, and she'd be wasting precious time on something that seems very selfish and silly (and fluffy), something she's been taught to be very afraid of. If she does though, I'm betting she'll feel a 1000 times better and everyone, not just she, will benefit from having her find some peace.