Friend's BC dx triggering my own fears

I have a friend - whom I have never had a very close friendship with, but we've been in the same social circle for several years - whose breast cancer was in remission for a few years, but she was just diagnosed with brain mets.  She's having surgery within the next couple of days.  Last night, I attended a prayer circle for her...a large group of her friends gathered in her living room (she wasn't there - she's staying in the hospital) and prayed for her, silently and out loud.  When we sat in silence for about half an hour, I focused on sending her metta -

"May you be safe.

May you be healthy.

May you be full of love.

May you be free of suffering."

And I visualized her at the ocean shore (where she loves to be), healthy, strong, full of energy, smiling, laughing, bright-eyed, surrounded by people who love her.  I made this image really strong in my mind.

But after a while, I couldn't sustain the focus on joyful prayers for her, because I noticed my breath was getting tight with anxiety and I started having scary thoughts about my own health.  I completed surgery and radiation for breast cancer 15 months ago, and am now on Tamoxifen, and overall I feel really healthy and good.  I usually think of the cancer as "over with," now that I've healed so well.  The Tamoxifen side effects are mild enough to not really be a problem for me.  I only see my onc twice a year and my surgeon once a year, and I hardly post here anymore, nor do I go to my local BC support group any more... I've been "moving on."  The cancer that was in me was stage 1 and a slow-growing type, and my Oncotype dx/recurrence risk is low.  So I have no logical reason to worry about a recurrence, and usually, I don't.  But during the prayer circle, I started having semi-panicky thoughts, like, I get my breasts scanned every year, but no one's scanning my BRAIN... I started worrying about cancer lurking in me in places no one would think to look.  And last night when I went to sleep, I had a horrible nightmare about being diagnosed with cancer again, going through treatment all over again, and then cutting off "cancer-prone" bits of flesh all over my body because I had read on the Internet that's what you had to do to prevent a recurrence.  It was such a yucky dream.

I feel so guilty for having such self-centered thoughts and emotions like this at a time when I really should be/want to be focusing on loving and supporting my friend.  Has this happened to anyone else?  Any advice?  How do you support a friend through her cancer journey, without it triggering your own cancer fears and anxieties?