Is this fair?: friend subletting house, me paying her mortgage, living together

Hi everyone,

My friend and I always had planned that when she buys a house we'll move in together, so that it's cheaper for both of us to live (I currently rent a flat on my own).

So the day has come and she's bought a house and will have a monthly mortgage when it's all sorted.

I just assumed we'd pay 50/50, as we're sharing the house together, e.g. if the mortgage is £800, i'd pay £400 and she pays £400 (and split bills 50/50).

But now she's saying that the deal is I pay her £800 a month, her full mortgage, like a renter (even though we'll be living together and it's her house), and she pays half the bills only. So effectively I'll be paying her mortgage off for her, while her outgoing will be 0.5 bills and she'll pay if anything in the house ever needs repairing.

I don't know if I'm being unreasonable in thinking that this is unfair on me. I don't feel comfortable paying her full mortgage.

I expressed these concerns in this conversation and she just said that she's giving me a really good deal and that I won't be able to find anywhere that cheap elsewhere, and said that I can't back out now because she's like already bought it and will have to start paying the mortgage soon, and she will have to find somebody else to move in in able to afford it

Does anyone have any thoughts on this please? Is this a completely normal thing, or are my feelings justified?

Thank you in advance!

[EDIT]

Thank you so much to everyone who replied and for sharing different viewpoints. As the cost for me won't be much different to where I'm currently renting and won't benefit me much financially, I decided not to move in with her. She is going to find somebody else to move in and pay her mortgage