How do I tell my mum I'm moving out?

Iv now been with my boyfriend for four years and we get on soo well that we really want to start thinking about our future a bit more. We are both nearly 22 and I have already got fertility problems so we want to make sure that we can live together and get on as well as we are now wen we are living together before we start trying for kids. We both have very sable and good jobs for our age we are not planning on buying just yet as we do not think that making that sort of commitment before we have lived with each other is right. We have been offered to rent a house for a great price and would not find a better deal any were. My mum is not impressed with the idea of me moving out at all. She doesn't know about the house that we have been offered she only knows that we have thought about moving out not that we have been looking. When we spoke about me moving out she went nuts and said that I was out of order. I really don't know how to tell her that I could be moving out within the next two months. Also I am worried about my perants financial position as they are not doing well at the minute and my pay really helps out in the house but I won't be able to do this when iv got my own household to think about I planed to try and tell her about the offer tonight but I just couldn't spit the words out no matter how I worded it in my head what can I do? Shall I write her a letter send her a text ? Any help will be good need it ASAP please any advise would be great x

Write her a letter. You can get everything out and know exactly what to say and how you feel without leaving anything out. Also, that's your parents they will be able to find a way to support themselves, this is your life. You start it