Absolutely not attention seeking. Be thankful OP, you've not experienced cancer, the fear of it returning never leaves, as does the feeling that despite being given the all clear, they've missed something and cancer is present.
For me, the whole cancer thing was surreal, not to mention for much, if not all the diagnosis stage, I have cancer question mark, rather than the statement, I have cancer. I imagine your mother is experiencing the latter albeit with I don't have cancer. Indeed, I've had cancer, chemo, much surgery, radiation and various other treatments. Had also had a few scares re. the cancer spreading. Even though I'm six months from the first milestone - two years clear, I know it's just going to make me worry more because cancer is a sneaky bastard and can come back. Sure, it might not, but, it can. I wonder if your mum thinks that if she still thinks about having cancer and living with it, it's not going to too much of an adjustment if it returns. Maybe, she's simply not processed it all yet and finds it easier to talk about still having cancer. I know on occasion, I find it easier to say I'm still undergoing treatment because I live with the knowledge that I have terrible genes/more fear than most for my cancer to return and all the women in my family have died of either my cancer or related cancer by 50. Just because one is 'cured' of the cancer that was diagnosed or there's 'no evidence of cancer', it does not mean there isn't a rouge cell laying dormant waiting to wake up.
Counselling is an excellent idea, as is your mum calling Macmillan or using a Macmillan drop in centre (found at most hospitals that treat cancer) to talk things over. If there's a Maggie's, they offer a post-cancer workshop, and I know Macmillian offer something simple. Your mother has been through the whole range of emotions including having to face her own mortality. Add in she's a mother and her fear of dying well before her time and you're still young is something that will still be on her mind and will always be on her mind. (I say this as someone whose own mother died of cancer at 50, when I was in my teens.
On a pertinent note, to those trying to diagnose the OP's mother with a personality disorder, ffs, don't be so ridiculous, and besides, until you've experienced the utter shit show of cancer plus the very real fear it's not all gone/will return any time, you really have zero understanding of how all-consuming the fear is.