Q. Is it true that we are defined by the choices we make? If so, once we chose God, will our other choices still matter?
The Bible does not tell us we are defined by our choices. Our choices in life will impact on our experiences, and our identity is, to an extent, formed from our experiences and our responses to those experiences.
But, no one has to be defined by their past. The Bible says, if anyone is in Christ they are a new creation (2 Corinthians 5:17). The old has gone; the new has come. In this life we can experience meaningful and lasting change, as our mind is renewed (Romans 12:2).
When we choose God, that begins a transformation. If we have truly taken Jesus into our life, it will change who we are. The fruit of His Spirit will begin to be displayed in our life. Our choices will change too, as the things we want begin to line up with the things God wants for us. Paul describes this as being set free from a slavery to sin, and becoming a slave to righteousness (Romans 6:18).
This is not an instant change and does not happen without some effort on our part. We will still face temptation, and initially, not everything we want will be what God wants for us. This is because we still have the potential to sin. But sin is no longer our master (Romans 6:14). We do not have to sin.
Just making a choice to accept Jesus is not the end of it. Following that choice, our other choices still matter very much. We must resist temptation and train ourselves to think the way God does. This work is made easier by the grace of the Holy Spirit working in our lives (Titus 2:12).