How Do I Cheer Up My Depressed Girlfriend?

All you can do is try mate, don't try and be a hero. Just try and see things from her point of view, be supportive, help her with whatever treatment she is on and most importantly remember that you are not the problem. She has a problem and needs help yes, but this does not mean that you are at all to blame. There isn't a single relationship without problems they just come up in different forms.

As for cheering her just try simple tokens of affection, flowers, hugs, kisses. They might do the trick, but then again they might Just get thrown back in ur face.

Hope this helps

You take her to the movies or dinner

You can't cheer up someone with real depression.  Real clinical depression is a medical mental health disorder and it can't be cheered up by anything other than a major psychological or neurological change.  Many people in this situation choose to use medication, but some use more personal method such as dietary change, exercise and some counselling or CBT with a psychiatrist.  CBT is Cognitive Behavioural Therapy.

If you love your girlfriend very much, you should be very supportive and help her through this hard time.  She might not thank you, she might be difficult, negative and down right annoying, but you need to stay strong and positive and make sure that you are part of her recovery.  Be part of that recovery by encouraging her to seek help, she won't want to because it seems more painful than simply hiding away.

Persevere.  This is going to be tough, but you will get through it.