hey leonie you just don't get it,
I understand your issue very well, only there is no solution but to use the provided tools in iOS 8.
It looks like Apple blocked large email attachments completely for any app. I cannot mail any file larger than 10 MB in any way from my iPad or iPhone, neither photos nor videos. all ill bring up this warning.
Since downgrading to iOS 7 and the earlier GarageBand version is not supported by Apple, you can only send feedback to Apple and report a usability problem and request a feature change.
Use this form: Apple - GarageBand IOS - Feedback
BTW: none of my friends or family members can receive attachments larger than 10MB. It is only our university network that allows to mail larger files.