OP, I know that you said that your mum will be upset but you will never be truly happy unless you start living life for you. I'm not here to encourage you to leave Islam, as you have clearly stated that you do not want to but you can still be a Muslim while wearing what you want. If you keep living to please your mother, you will look back on your earlier years and feel that you were wasting your time; eventually the way you dress is going to get even more difficult, considering that you already have a problem with it.
I agree with the people who have advised you to wear light but modest clothing like maxi dresses with cardigans. If you don't wear a hijab while sticking to that dress code for at least a while, hopefully your mum will be more at ease.