In need of some advice...

Hi, I'm a 17 y/o guy from the UK and I have a problem that I can't solve myself so I'm hoping you all could help me.I'm gay but have a girlfriend. That's my problem. I have a girlfriend because I used to think I was straight but recently realised I was gay (I was very, VERY naive and it never dawned on me that I was gay). I realise there is only really one outcome to this which is to split up with her. How do I go about this? We've been dating for about a year, she's totally in love with me and I really do care about her just not in that kind of way. I want to keep on as good terms as possible with her but I just don't know how to do that or how she'll react when she finds out her boyfriend is gay.Any advice is gratefully received and I'll answer any questions that might help you help me, thanks in advance.BTW this is my first ever Reddit post despite this account being a throwaway 😂