also - regarding the mistakes you've made before and since you accepted the guidance, know that there is no person on earth that doesn't make mistakes, those mistakes should make you realise the depths of evil and make you a better person, don't become weak and faint-hearted, just regret making the mistake use it to your benefit and see it as a reminder of how evil evil really is,
i remember this hadith often when i've done something wrong and feeling ashamed to stand before God in prayer,
“All human beings are sinners; the best of the sinners are those who repent.” (Ibn Majah, Zuhd, 30)
“If you were not to commit sins, Allah would sweep you out of existence and would replace you by other people who would commit sins, and then would ask forgiveness from Allah.” (Muslim, Tawbah, 9, 10, 11)
Islam is a religion of nature. In Islam, it is accepted that man might commit sins and he is taught the ways of avoiding committing sins. The way of getting rid of the sins and crimes that have been committed and being purified of spiritual impurities is repentance. Thanks to repentance, a person becomes free of the sins and mistakes; he becomes as clean as if he has not committed those sins. As a matter of fact, the Prophet said the following regarding the issue, “A person who repents from his sin is like the one who has never committed that sin.” (Ibn Majah, Zuhd, 30)
38 : 17
17. Isbir AAala ma yaqooloona waothkur AAabdana dawooda tha al-aydi innahu awwabun
Disregard what they say for the time being and remember our bondsman David, the possessor of strength who constantly returned to Allah!
Prophet David (may peace be upon him) is described as being
awwab which is someone who constantly goes back. Ibn Kathir says, ‘He would go back to Allah in all his affairs’. Awwab is used in the Quran mostly to refer to the repentant. Dawud would always repent to Allah for anything great or small. Ash-Shawkani said, ‘He would constantly turn back to Allah from anything that Allah disliked. This is only possible if someone is strong in his religion’. This last part of the ayah is a causative statement. In other words the fact that Dawud constantly turned back to Allah was the cause for his strength in worship. Such was the state of the Prophets in their relationship with their Lord.
remember that there's always time to regret past evils and forgiveness is open till death - and you don't know when you're going to die (could be as you speak just as it could be forty years away) and continually falling into the same fault means that your brain has built strong paths which increase liking that deed and hence stimulate your opiates, these paths are always there but become stronger with repetition and weaker with time when you avoid even thinking about it which can cause a trigger effect. i hope i have made a positive contribution and not rambled too much, if there are any mistakes they are mine, if there is any good it is from God Almighty
peace