Personally I use self-deprecation for humour, or if I feel uncomfortable taking a too assertive stance - when I do get down in the dumps distraction is the best thing (Though my mum had to indicate this to me, and then I had to have an experience of it making me feel better to really let this idea stick. Before I felt uncomfortable from disengaging part of my emotions, gaining feelings of a secure, tangible identity from them. It's healthy for fours to recognise that the self is a fluid, ever changing thing, you are whatever you are in each and every moment) - not to say it always has to be out, away and never talk about stuff, it's good to get stuff of your chest, but if I'm in a rut/slump, then a change from that headspace freshens me up and gives some distance from feelings or thoughts I felt a bit trapped in - if the new environment is neutral, then I can think 'hmm, maybe I'm getting worked up?' It can be as simple as come down stairs and have a warm drink watching rubbish (Or something good) on the TV with others around, or something much more active. Positive social experiences are good too, as they raise my confidence, and reinstill understanding that I'm cared about, maybe even show to me that I'm being a bit meladramatic, as well as giving me some distance from the negative feelings.
As for changing negative beliefs, it depends what they are. If there is some fourish significance tied to the faults in a weird kind of way, perhaps state you love/care about her just as she is - give her the sense that you enjoy her company, that tiny mistakes or any uncomfortable moments and such like won't change that, though don't lie. It's possible (I don't know though, you'll know better than me if she is) she's uncomfortable in her own skin and so feels others won't be comfortable around her either.
As for your lower Fe, if it comes up/seems relevant, let her know this about you, if she doesn't have that feel of your character already. Perhaps get together with her/some others and talk about personal traits or how you view the world so she can see you have a sorta different world view.
I know Fi/Te can be a little foreign to me (I know you're not Fi/Te, just giving it as an example), learning about the MBTI has helped me understand what I can't see on the surface, and what works in ways different from me. But expressing something of the differences in people which she might not be aware of might not be necessary, you'll be the best judge of what you think would be beneficial in terms of her understanding.