But sometimes she worries that she will lose me or we will grow apart, which leads to other insecurities. These feelings snowball at times. I am working really hard to reassure her but that doesn't seem to be enough. She knows she is not trapped into this. Any suggestions on how best to help her? Perhaps time? Perhaps we slow things down?
Remember you have lost something. Even if things are going great, you have lost it "being just the two of you."
She could have stepped in at stage 4 in the stages of grief
and need support through it. There was a similar thread recently here
If her jealous is coming out of "fear of abandonment" or "fear we will be losing touch"... could perhaps read this together
and talk about doing more page 5 and page 6 things in addition to what the previous posters suggested.