My girlfriend is considering breaking up with me

Me (15m) and my girlfriend (14f) have been together for coming up to 4 months now and although that may not sound too long, the connection and love I feel for her is so strong and I genuinely won't love anyone else the way I do her. She had the confidence to tell me about something in her personal past because she trusted me but I've thrown all her trust out the window by speaking to her mum about it. She has said she now no longer trusts me at all and never wants to tell me anything again. I can't express how regretful I am of my actions and would do anything for her to trust me again. I really don't want her to leave me because I only want her in my future. This is my first serious relationship and a lot of the time I think I don't really know how to handle a relationship this serious, talking about futures and family etc. We met on a holiday and kissed and said I love you and all that on the first day we met (my first kiss, hug and pretty much everything). She's now saying that she really doesn't know what to do with our relationship because it should be built on trust which I have broken. What do I do in this situation and can I save this relationship?

Update: She didn't break up with me and we're talking about how we can build us back up. Thanks to you all for the advice!