In the Narada Pancharatra, it is said bhakti begins only after we give up our material designations that is connected to our body and mind. As long as we are connected to the body and mind, any devotion we perform is mixed devotion (mixed with karma and knowledge).
When a soul engages in service to Krishna from his constitutional position as servant, then that service is called bhakti. Any service to Krishna thinking I am this body/mind becomes mixed. So this mixed service is called sadhana kriya as opposed to sadhana bhakti which stems from the platform of servant to God (soul's constitutional position). This difference between sadhana bhakti and kriya is indeed subtle. There is no definitive line. If we perform bhajana such as sravanam, kirtanam and archanam with the end goal to benefit the self (in the form of the body and mind), then that is sadhana kriya. But if we perform the same bhajana with the end goal to please Krishna ( as a servant of Krishna) then that becomes sadhana bhakti.
Sadhana kriya is considered pious activities and can help cleanse the mind of bad qualities (anartha) and establish the soul on the mode of goodness platform (satva guna) but such pious activities is still temporary as it is on the platform of the mind. If we want the same activity to become eternal, then it has to be done on the platform of the soul, which is to serve to please Krishna in a mood of servant.
This inner mood that I am servant and my service is for the pleasure of Krishna dictates our growth in spiritual life. Anything not this, we can chant 64 rounds daily but still we will not progress to the stage of bhava or prema. That is why (at least this is a revelation to me) that many devotees who come to this platform of bhakti fall down or leave because they have not given up their material identity. They have not cultivated the identity that I am servant of God and my service is to please Him. One who cultivates this thought will not fall down.
Therefore, It is paramount to cultivate this inner mood of servant to Krishna and perform our sadhana in that spirit of servant to Krishna in order for us to not fall and indeed make progress. Krishna says in the Bhagavad Gita that this is possible only by His mercy! Ultimately therefore, our ability to perform bhakti is dependent on the mercy of Krishna. If we do not have that mercy, we will be performing sadhana kriya (mental platform) and not sadhana bhakti (soul platform) and there will be every chance of a fall.