If your SO is experiencing depression, then she needs to seek professional help. The most you'll be able to do is be supportive and understanding as she goes through treatment processes (whether they be just talk and behavioral therapy, or medication, or more). This includes being nurturing and asking her what she needs from you in a moment to help her get through an episode. I know you say right now she is dissociative with you, but in the future with help, she may want to reach out to you for snuggles and silly movies, as one example, when she's feeling really low.
It sounds like she has general insecurities for a young woman and they are being exacerbated by her depression - this is normal, and with enough help she can, in theory, get a handle on it.
As someone suffering from PTSD while also in a LTR with my husband, I can say (something that might be an unpopular view) that sometimes, mental illness doesn't necessarily warrant a breakup. If the two of you still have genuine romantic feelings for one another, it may be worth while to stick it out while she gets help - but ONLY if she gets help. If she refuses to do so, I wouldn't stick around. Mental illnesses are tough and really take a toll on the person diagnosed, and all of their involved loved ones, but being ill does not lesson the value of the person.
Ask her if she'd be willing to get help, or if she's worked with a therapist/psychiatrist in the past, if she'd be willing to begin working with one again. Remember, just because you (as in she) find a psychiatrist/therapist who has an open appointment book for right-away doesn't mean they will be a good fit for her. It can take time to find the right therapist/psychiatrist to work with, and that could be anything from a handful of visits to different professionals, or even months of searching. Rarely, she'll click with the first one she visits, and that's fine too! The important part is encouraging her to seek help, and follow her therapist/psychiatrist's advice. If she is put on medication, she needs to stick with that medication. She should never stop taking it when she feels better. Mental illnesses do not go away, they are with you for-life. Managing them is a regiment, requiring constant upkeep to learn to cope with what is going on in your head and apply those skills to your daily life until management of your illness becomes second nature. Even then, that doesn't mean it's okay to give up on medication, and if one is in active therapy, they should check in at least once per month (after the initial stages of weekly and then every-other-week visits).
It is entirely possible to go on to lead a happy, healthy, balanced life while coping with a mental illness. It doesn't make someone less than a "normal" "healthy" person, it just means they have to work harder to be happy and well adjusted.
Finally, if she does start going (or has gone in the past, and had this response) and after a few sessions starts to feel genuinely happy and wants to quit - encourage her not to. With mental illnesses of any kind, just because you're suddenly feeling better, or have been feeling better consistently for weeks, with little to no outbursts/episodes/symptoms does not mean you're ready to quit. Only a therapist/psychiatrist will be able to say when she's ready to reduce how often she visits with them.