What if she still loves you? I think you are acting too weak and not "fighting" for her. She is getting fed up at how you put up with the way she treats you. She is trying to get you to be tougher. Did you ask her why she is unhappy when she said that or did you just get upset and walk away? She may have said that to get you to react, say something back, tell her you care for her, something...instead, you now sleep on the couch and are agreeing to move out when you can afford to do so.
Are you telling her that you don't want to move out, that you want to work things out? If she is always leaving you and breaking things off, then coming back to you whenever she feels like it, that causes her to lose respect in you. Do you tell her that's not acceptable and that you won't take her back, so that she doesn't keep doing it? YOua re acting like a doormat, letting her walk all over you, then you get upset that she is unhappy, and you sound like it is coming out of nowhere and you are shocked, since six months earlier she had written you a love letter stating she wanted to marry you and all that.
Well, feelings can change minute to minute, with each word and action! There is no guarantee that you can do and act however you want and that you will always have her love. You say she always wants space - for dating that other guy from work? from getting away from the doormat behavior? I think it is hard for her to just abruptly end a 4 year relationship, so it has been ending over the past several months.
Usually once a woman stops having feelings for a guy, it seems sudden, but it has been building up to it and there is in my case no negotiating back. if I no longer have feelings for you, it's completely over. So maybe she is now entering that stage and you are confused as to how "suddenly" she can be so indifferent.
You have to work on your self-esteem for sure and talk to her, tell her you want to make her happy, LISTEN to her, find out how she is feeling and why. I think she is frustrated that you don't inquire and are just putting up with whatever she dishes out your way. You shouldn't treat yourself so badly by putting up with someone treating you so disrespectfully. Maybe the futon will stiffen your spine a bit

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