Almost nobody came to my [20M] girfriend's [21F] 21st birthday party :( What do I say to her?

So my girlfriend just turned 21, and has been planning a big party for literally months. She had a great theme, just got a new apartment to have it at, and invited most people she knew. She was so excited and was talking about it for months. around 40 people were expected, and it was going to be a great time. Well, the party was this Friday night, and barely anyone turned up. about 14 people total came, but that’s not even all at the same time. There were probably less than 10 people there at any given time.

It was heartbreaking to see her there in her dress trying to hold a smile while everyone was just sitting around kind of awkwardly waiting for it to pick up. She wanted dancing and fun and good memories, and it was just awkward and quiet. I tried to do the best I could, and have for the rest of the weekend since, to cheer her up, but there’s only so much I can do in such a situation. She is so sad and let down and says she can’t help but have her feeling hurt and take it personally. I slept over at her apartment and seeing her look at the party decorations and mostly unused party favors was so sad.

I tried to give her a good day yeterday (saturday) and we went out and did stuff, but barley talked about it at all, kind of elephant-in-the-room kind of thing. Eventually I asked her at the end of the night if she had been thinking about it and she started crying and said how she had such awesome hopes for this party and never even considered no one would come. I tried to cheer her up after that and we made some food and ended up having a good night.

I feel so terrible. This is one of the nicest, sweetest, friendliest girls I’ve ever met, and she didn’t deserve this. It is absolutely eating away at me, and I’m sure at her too. What do I say to someone in this position? How can I make it better? How can I make both her and myself stop feeling so bad over this? How can I convince her that she does have friends and that she isn’t unliked? I have already been thinking about throwing her a surprise party next year for her birthday and making it as big as possible, yknow stuff like that.

Thanks reddit, any advice is greatly appreciated.

TL;DR; very few people turned up to my girlfriends 21st birthday party that she had been planning for months. She didn’t deserve this, and it is heart breaking to see how it made her feel. What can I say? What can I do?