Even though you did your best, it is not enough and even worse, your opponent manages to bring you close to an edge. Your only defense is to interrupt your efforts. You recognize that this is not the right way to make your opponent cum, so you give up and start making better plans.
With wonderfully soft breasts like that, it would be a shame to only let your hands and mouth experience them. You take out your cock, slide it between them, and tell your partner to hold their breasts together while you begin to fuck them gently.
Even though you used your best fingering skillls, it is not enough. You recognize that this is not the right way to make her cum, so you give up and start making better plans.