Even in situations where people have generally healthy self esteems, insecurities will pop up eventually. Jealousy will happen eventually. There just isn't much of a chance for everyone in a triad (or more-ad) to feel equally enamored and invested as the others all the time. It isn't realistic to expect this and it is a big part of why the insecurities just naturally come into play at some point.
You could say you're equally invested in both till you're blue in the face but you cannot promise it will stay that way. Not for you or them.
That this stumbling block came into play so quickly and isn't the first time it has for them - you could take it as a sign of them just not being ready for what you're looking for or that they may not really know what it is they are looking for.
Perhaps share with her what you posted here as a second attempt if it will make you feel like you tried your best. However, it could also be seen as a bullet dodge that things didn't work out.