1- Even if the sin is repeated, Allah still forgives if you repent. As long as you feel regretful and feel bad about the sin, then Allah forgives. Yes if it reaches a point where the person no longer cares about the sin and doesn’t feel bad about it, then this is a dangerous situation and one’s sin may not be forgiven if this person dies in this state. But as long as you are sincerely trying to stop this sin, and you genuinely feel bad about and ask Allah to forgive you, then He forgives.
2- Come up with creative ways to stop the sin or at least reduce its frequency. For example, do your best not to spend any time alone, and keep yourself as busy as possible. Avoid anything that triggers committing that sin for you.
3- Remind yourself how merciful Allah is. How good he’s been to you. How many blessings He’s given you. Why would I sin against such an amazing Lord? Let this thought give you the strength to stop the sin. And remember when you say no to a sin, immediately Allah has granted you an amazing reward.
4- Yes, even if you have had such a history, Allah can forgive you and purify your soul, and give you that status. It is definitely achievable. By the way, one hadith by Imam Ali (a) says, “The one who can say no to his desire and remain pure when he is able to commit a sin, his status/reward is not less than a shahid.”