This question tends to be up for debate depending on who you ask. Within psychology alone there is not alignment on this issues and schools tend to differ on their treatment on the past and its influences on an individual. Perhaps the simplest answer would be as much as you want or need it to have. Of course this is an oversimplification as a traumatic childhood may absolutely produce significant hurdles in development. That being said, no one has had a perfect past. All of us have experienced dysfunction within our families, friends, or relationships.
The larger issue here is blaming the difficulties we are having now on our experiences of the past. We tend to hear people blame their pasts for their shortcomings, but rarely hear them blame their past for their successes. We also tend to take credit for our healthy behaviors while blaming the unhealthy ones on someone else.
That being said, the past is very influential in our development and has brought us to where we are now. Everything that has happened and everything we have done have played a significant role in forming our current selves.
We are all well aware that we cannot change the past. Why then, is so much energy wasted living in the past? We’ve all replayed experiences in our minds, asking “what if” and “I should have done”. This is just another way of missing life in the present moment. The past, including even the time you’ve taken to read this post, is gone. It is inaccessible and unchangeable.
The best way to leave your past behind is to accept and appreciate it. Stop the guilt, hate, regret, wishing, and fighting. Accept there is no changing it and see that it’s only stealing your time from you now when you obsess over it. Reward yourself for being who you are; for surviving. Appreciate what the past has taught you and the tools it’s given you. If you hate your past then appreciate it for showing you what does not work in life.