Me [16 M] with my girlfriend[17] 1 year 2 months, thinks I'm talking to someone else when I'm not

Hi guys, So me and my girlfriend have a really strong relationship most of the time. It can be quite stressful because she is currently grieving and my Father is terminally ill. This aside, we try our best to always be happy.

She was on my phone (which I had no problem with) and she saw that I had a conversation with a girl. Perfectly normal conversation, nothing being said that shouldn't. She, (who also has anxiety and depression) assumed the worst and is now going crazy at me, threatening to break up with me etc, saying I'm worthless when I've put everything into this relationship:(

She's said "I'm done" but I know for now it just means she wants me to piss off...

I don't know how to handle this and what to do :( I know it sounds a little stupid to most of you, but this relationship is honestly all I have left and I know I would struggle more than anything without her.

Any replies mean so much to me, thanks!

tl;dr: Girlfriend thinks I have been talking to another girl when I haven't! It was a normal convo. She is threatening to break up with me..What do I do?