LewsTherin:
Women are very emotional creatures. Your mother is one. She can't see beyond her hurt to realise she is hurting you. In her mind, she rationalizes it as only wantong the best for you when all she feels is a non-existent betrayal. I should know. I have been independent for over 10 years and my mother still can't seem to accept the fact that I am my own man with 2 kids to boot. I guess being the last child of the house who was always a "good boy" has its disadvantages.

What I have done is to reduce my interactions with her. It hurts, yes but it means I get things done before she knows about it. By then, she can't influence or demand I follow her own opinions. Deed is done already. The result is a relatively strained relationship but at least not a bad relationship as it would have been if I had gone full rebel, nor a toxic relationship as it would have been if I pandered to her every wish.

I believe you have to do something similar. Your mother is comfortable so she doesn't need your money. It is not about the money. It is about loosing her connection (means control to be honest) to you. So this is what I suggest. Move out. Get yiur own place. Don't tell her before you do. Don't bring your girl anywhere near your mum. Don't make it obvious to anyone- siblings inclusive- that you have a girl. Else your mum will believe the girl has taken you away from her. Visit your mum on a regular and fixed schedule - like every sunday, or sonething. Do not fail. If something happens to make it nit happen, see her the next day. You have to create a space between you two and only after she has accepted that should you even begin of thinking of letting her know you are getting married. Note. Marriage, not girlfriend. Let her express surprise that you had a girlfriend in the first place rather than she telling you some girl is not good to have as a date. Once thay stage is reached, you fiancee should cime for your regularly scheduled visits once in a while. Not all the time. Once in a while until she warms up to your girl or until she is resigned to your decision.

It is better for her to say you don't keep her informed as to what you are doing than for her to get into a grudge match over the things that you want to do.

My opinion.

Waw.. Thank you very much for this. I've always thought about something similar.... Executing it is what makes me sceptical. Wouldn't want to do it wrong.
But I guess I have to give it a good shot.