Does true love last only after marriage, or can love that began before it continue, even if it was accompanied by some transgressions?

The beneficial and lasting love is love for the sake of Allah. Every friendship not for the sake of Allah will turn into enmity on the Day of Resurrection, except for what was for Allah. The cure for a heart attached to a woman is to marry her if possible, for the Prophet, peace and blessings be upon him, said: "We have not seen for two who love each other anything like marriage." If something forbidden occurred between them, they must repent to Allah, for repentance wipes away what came before it.