I don't think YANBU to feel this way, but you would be unreasonable to let her know that you do, as it might make it difficult to get your friendship back on track.
I understand how you feel, although my situation is slightly different. My closest friend and I have known each other for 20+ years and have been through all sorts together (boyfriends, marriages, babies, illness etc.) and have always spoken at least once a week even when we've been busy and have seen each other at least once a month in this time.
My other closest friend was a Mum from school who had children in the same classes as my 2 DDs. The 2 friends had never met, but of course I had spoken about each of them to the other over the last few years (always kindly, luckily as it turns out for me...) Both friends decided to send their children to the same school from last September so I put them in touch and facilitated a meet up so all the kids could get to know one another before the new term began. This was at the start of August and I have seen neither of my friends since.
I have called and texted both, but apart from a few quick texts to say hi or to reply to my messages, I haven't been able to pin down my oldest friend for a proper chat or to meet up in over 4 months now, which makes me sad. What makes me really jealous though, is seeing all over Facebook that these 2 friends have now become bosom buddies and seem to do everything together with their kids. All the things we used to do, they now do without me and my DDs. It makes me sad, and as I said, jealous - but if I were to say so, it would make me seem mean and petty; after all, I introduced them, talked them up to one another and hoped they would become friends. But I have to be honest with myself and admit that I had hoped that encouraging them to be friends wouldn't be at the cost of my close friendship with them both.
I think you're lucky to be in a position where you have the opportunity to rebuild your friendship - I would grab it with both hands and try very hard not to let your jealousy about her friendship with this other person affect your relationship with her. I know it feels rotten, but it sounds like you had a great friendship, which you clearly valued so do please try not to spoil it for yourself, though it may seem easier said than done!