Hey,
I don't want to be the devils advocate but i understand this situation quite well too. I had a flatmate who became incrediably dependent upon me, she would come crying to me constantly over the stupidest things, wake me up in the middle of the night to talk, i'd have to walk her to the drs at stupid times in the morning because she was afraid she'd faint, she'd complain about the stupidest things like her theighs hurting after she did exercise thinking she'd twisted a msucle or something and no matter how many times i told her it was natural she'd keep complaining - it literally drove me insane. And actually to become very ill. She too made every single thing about her to the point where i spent so long focusing on her that i was neglecting myself badly.
I probably sound incrediably egotistical above but that is literally what happened because i did care about her. However with hindsight i realise that i actually brought a lot of it upon myself. At the beginning i would sympathise with her and spend a lot of time and energy trying to make her feel better, i almost wanted to 'fix' her. I think i didn't let her muddle her own way through and maybe by actually giving her someone to talk to made her more self-pitying and self-indulgent. I basically made a mistake and didn't realise how much people with depression need constant care for extended periods and that was not something i was able to give her.
I feel that maybe this is something that happened to you too? You said that at the beginning you felt so much for her and were there for her whenever and wherever - thats a big committment to make even to a family member. Prehaps like me you gave too much to start with thinking that these problems after a while would decrease when she has talked about them? Prehaps compassion also became something else - like you had the ability to help her like noone else did? This is something i have come to accept was part of my thinking, mainly due to nievety i think.
So although i understand you frustration entirely, it is not suprising that she reacted strongly to you 'suddenly' at least in her eyes denying her any interaction at all. Afterall you were always willing to listen before so in her point of view - what happened? Was there any prior indication of you getting annoyed and attempting to change the conversation or when she got onto a topic you didn't want to talk about giving big hints that you weren't prepared to listen - like quickly making an excuse up why you had to leave?
To be honest and this is just my opinion i don't think you needed to write her a card to explain anything. That makes things very final - very black and white and prehaps not in the best interests of either of you. I understand that someone asking for your attention all the time is hard to be polite and deny it but with my friend i learnt to consciously change the subject if i was with her or make an excuse to leave when i wasn't able to deal with her so that i still could if she needed some support at times. I've learnt through a bigggg mistake that the issue was less to do with her (although of course she has issues) but more MY expectations on what i felt I needed to do for her, and me not securing some boundries regarding our relationship.
I honestly don't think she is trying to make you feel guilty or manipulate you, i think a lot of this happened for various reasons and if we put ourselves in her shoes (no matter how annoying she is or that may be) - all she saw was someone giving her loads of compassion and then taking it all away for no reason. Remember she doesn't know your triggers or issues.
Anyway i hope i didn't offend you, this just fitted in with a lot of what i have been through recently and thought maybe i could help share my insights. I do understand how annoying and frustrating it can be when people do cling! Just in case i haven't been explicit in that!!