Really good questions. I hope we can help you.
When an individual gets born again (check this link if you are not sure
Are you absolutely sure you are born again? - Christian Life Frankston) he/she receives a new spirit. The spirit is the real you. We are born with a dead spirit, dead to God and usually dormant. Instead, we learn to live from the soul - what we think, how we feel and what we decide. The activity of the soul is at enmity with God. That is because the soul operates according to the principle of good and evil. Adam and Eve were warned against choosing to live by the tree of knowledge. They disobeyed. The rest is history.
The new spirit is alive to God and is where Lord Jesus is able to dwell. We receive direction from God in our spirit man. Very often, there is a dramatic change. I knew someone who was a karate champion. He gave it up the moment he got born again. His conscience warned him that Karate is wrong. He still had to choose to give it up.
Other issues may take time to resolve. We have the answer in our spirit man, but we may still be reluctant to follow the answer. For example, I was psychologically addicted to alcohol. It took me several months (and a really bad time) to realise that the alcohol was not doing me any good. I was no longer dependent, but it was a part of my lifestyle.
Some sins seem to drop off immediately. Others take time, especially if we don't see much wrong in the sin at first. Beliefs do not automatically change. People with mental issues need to be very careful about stopping medication. The Bible says that we need to be transformed by the renewing of the mind. That means that attitudes need to be compared with the Bible and adjusted accordingly. It is a process - never automatic.
Christians sin. God's standard is Jesus. No one matches up! One day we will be like Him, but not in this lifetime.
Some of us wrestle with doubt. Before I was saved, I was sure that I was a Christian. I had absolutely no basis for that belief. After I got saved, I had many doubts for years. Eventually I decided that God's word was true, God was faithful to His word and God was perfectly good and just. I started to "doubt my doubts" as a friend of mine put it.