Is repeated repentance from masturbation—in a state of certainty that one will return to it—considered sincere repentance, and do sins delay marriage?

You must repent whenever you sin, and do not despair if you repeat it, for Allah is Forgiving and Merciful. The one who repents from a sin is like one who has committed no sin. The conditions for repentance are: abstaining from the sin, resolving not to return to it, and regretting having committed it. Performing two rak'ahs for repentance is good. Persistence in sin causes deprivation of provision, for the fear of Allah is a cause for provision. Repent with sincere repentance and resolve not to return to sins. Strive to improve your deeds, associate with the righteous, adhere to dhikr (remembrance of Allah) and du'a (supplication), and have a good opinion of Allah.