Move on from R, by cutting contact. Think about obvious giveaways she was manipulating you, and terrible in general.

Failure is for most people natural part of process of seeking their forever partner. Accept reality for what is, take some time as single to get over it, and start dating again.

S has as, you noticed, already moved on from you. Her relationship might fail at some point, but don't think it will necessarily mean she will want to immediately get with, in fact she might not want you anymore in general. Just don't take her affection for granted.