all the sefrei chasidus are full with this concept.But if you wish to go further back than that ,you see this clearly in the neviim.In which even when terrible sins were committed,Hashem keeps on saying again and again and again ,do teshuva and I will accept you back with open arms.also look at the hadamah of sefer chofetz chaim.and all spead out in his writings.in shimas haloshon as well.that it will take time to remove the sin of lashon horah.Its not easy to uproot.But as as long as you admit your flaw,ask for help , put in the effort,Hashem will forgive.You see it by Menashe as well.You see it by Yerovam as well.You see it as its brought in seforim even with a person like Acher who basically in Shomayim he was told to take a hike and get lost.But as the seforim say,if all of these people would have pushed themselves and humbled themselves,they would gave been forgiven despite the terrible sins that they committed.So yes,God is more upset that we dont take advantage of the gift of teshuvah more than He is with the sin itself.Though I have trouble comprehending it,the executioner of R Chanina Ben Teradyon,Nevuzardan, despite being mass murderers had thier teshuva accepted.The first a bas kol said he is going to Olam Habah,the second was accepted as a Ger.So go figure!!!
the key being that the voice that tell the addict that all is lost, and he will burn in hell no matter what, is a false voice meant to undermine him.If he does teshuvah,takes the steps towards recovery,he can truley become a great person no matter what he has done in the past.
and now,TZ,not in the context of my previously deleted post , I will reiterate the same thing I said before.The fire of machlokos burns higher and destroys more than the fire of arayos.I leave it to you to find the "citation needed" for this. One is simple the fact that the 3 real big NO NO's got us back the Bais Hamikdosh in 70 years while as the hakdamah of the sefer chofetz chaim writes clearly,its the sinas chinam that has all of us stuck in this galus for over 2000 years.
we all know that the Chazal say this.But Im sure when the Yidin first heard it from Chazal they must have been shocked.as they probably were when they heard that 24,000 talmidim of Rav Akiva died because of "lo nahagu Kavod Ze Bozeh".Its not the first thing that comes to peoples minds that they need to correct.There are much "worse" things than that.But chazal tell us that this sinas chinam is the worst of all