Pretty much this. Exes can't be friends anyways. One side will always want more, and neither party that wants more will enjoy settling for the scraps of affection and how your former gf/bf loved and adored you as a lover, but now you get this basically irrelevant attention as their relegated friend who gets barely any calls or texts at all ever again. Or if you're together hanging out they treat you nicely, but not with the same interest or attention. You or she will never be satisfied with that. It's not fair to either side honestly. Physical attraction never really disappears between exes unless one lets themselves go. Emotional and spiritual goes for sure, but physically it can always be reunited. So one side is gonna be frustrated as hell that all they get now is the same bs they would get from one of their friends. It will never work. Men don't hang out with women one on one that they don't want to sleep with and date, and don't invest their time into one that they don't have that pursuit of. Women want security and a family so they don't waste it on past ex bfs vs a new option which they have plenty of.

lol @ trying to keep a platonic relationship with an ex
maybe you can ask her if she's okay with being strictly fwb

You cannot trust an ex.