As we know Imaan fluctuates Its like a plant who needs water, nourishment. Without Dhikr, Salah, distancing urself from bad company it'll become stale n weaken. Don't become dishearten it can be fixed with the remembrance of Allah.
Start back from the basics, Tawheed! Ask urself ur purpose in this life, why were u created? Its to worship Allah alone.
Re-new ur intention n be sincere, the best way to purify ur heart is with salah InshaAllah. Understand as much as u can of the Quraan, when u read it read it with an open heart n in the hope to receive guidence from Allah.
Listening to a lecture doesn't necessarily change anyone, it's the willingness that comes from ur heart that can bring the change Allah willing. May Allah guide u ameen. You have to want this, need it, make it ur focus. I will also remind u to make dua, plenty of dua. Pour ur heart out to Allah, ask him to guide u n distance u from all falsehood ameen.
“Truly, God does not change the condition of a people until they change what is in themselves.”