When you love a girl who is depressed there will be extremely difficult days for both of you. There will be freak outs and break downs and blow ups. When she's slid to the floor, crying about not ever feeling good enough, get down there with her. Don't try to tell her she's being irrational. That will just invalidate her emotions and make her feel sorry for every crying about it in the first place. Get down there with her and wrap her in your arms. Tell her that she is always more than enough to you. She may not believe you but she'll take comfort in knowing she's safe with you. There will be days when it seems like nothing is wrong. Not a hair seems out of place, but there could be thoughts building up all day and stress and anxiety and it could all burst out in angry words she doesn't mean or tears she can't stop. She feels horrible that her brain is doing this to her and affecting the one she loves the most. Pull her into your chest and tell her she's okay. Tell her that you'll protect her always and watch her melt into you. The worst thing you can possibly do is get angry with her for breakdowns caused by mental illness. Believe me, even though she shouldn't, she probably already feels bad about having one in the first place. Be steady. Be protective. Be her hand up. Be her partner. Be her equal. Be her safety.
You'll start to notice things like she plays with her fingers when she's nervous or she bites on her lip when she's thinking something she feels she can't say. When things like this happen, Put your hand on her leg. She wants the reassurance of your touch. Tell her you're right here for her. Always pay attention to her because a lot of the time she won't tell you directly that she's feeling down or feeling uncomfortable or anxious. Love is such a scary thing when you're mentally ill. Feeling like the way your brain works will eventually push all the people you love away is a feeling that doesn't really ever completely subside.
There will be days that are not on a down swing. She'll laugh and joke with you. She'll get out of bed no problem and these will be great days. But just know that her brain is still running in the way that it always does. The same things will get to her. Looking out for and loving a depressed person is knowing them better than you ever thought you could know a person. You should know the difference between the look she gets when you tell her you love only her and she's having a day where it's hard for her to believe that and the look she gets when you tell her you love her and she's having a day where she knows it without a shadow of a doubt. After a while she'll begin to show a small smile and say "Thank you" after you compliment her rather than disagree with you. She'll feel more comfortable in the fact that you love her. But she'll probably still question it and ask you just to make sure all the time.
When she can't get out of bed, tell her that if she's not ready, you won't make her. Help her to feel like the day is worth it. Let her know how happy she makes you. When she breaks down and starts talking about things that are old and familiar and scary to her, hold her tight and reassure her that you're never going to let anything bad happen to her. She needs to know that someone doesn't want her to hurt as much as she does.
She is always going to be different to deal with than someone who doesn't have depression. She's always going to need a different kind of affection. She's going to need someone to turn to when she can't control her own thoughts. She's going to need you to quiet all the voices in her mind that say you don't love her. She'll need to be reminded that you do with all of your heart. She'll feel like she's high maintenance sometimes and won't stop apologizing to you. Don't let her. All she wants is to feel like she has nothing to apologize for. She wants to feel supported and taken care of. Never let her forget that you love her and everything that comes with her. Let her know that no part of her is too much for you. Always validate her feelings and emotions and always remember that even when she can't bring herself to believe that you love her, she loves you more than anything and she never thought that she'd be lucky enough to have someone as caring and selfless as you be right there for her no matter what. Never let her forget that you always will be.