Maybe you shouldn't have changed jobs.
Maybe you shouldn't have bought that new house.
Maybe you shouldn't have moved to a new city.
Maybe you shouldn't have taken that big promotion.
What do you do then? It's always easy to look back and criticize yourself for the decisions you made years ago. We've all done our share of second-guessing. It's normal, it's natural, and to a certain extent, it can be useful. But at some point you've got to move on.
I received a note recently from someone who said something like this (with identifying details changed):
In the early 1980s I left a very good job in Canada to move to Spain because I thought the Lord was leading me in that direction. But it never really worked out like I thought it would. Along the way I got cancer, endured treatment, and now am much better. I decided to move back to Canada where I am a lot older than when I left. And I wonder to myself, Was I wrong to leave Canada in the first place? How can I trust God with my future if I secretly think my past decisions were wrong?
On one hand, there is no way to answer those questions because I'm not aware of any verse that says, "Stay in Canada" or "Move to Spain" or "Move back to Canada." So from that perspective we're not talking about a sinful act. You could leave, you could stay, you could return, or you could move to New Zealand and it could all be within God's will for your life.
Life is short for all of us, and if any of us had a chance to do it over again, we would probably make some decisions differently. I know I would. But that choice is not given to us. Too much introspection about the past makes us tentative about the future. It doesn't matter what would have happened if the writer had stayed in Canada because no one but God knows how it would have worked out. The thing that matters is that he is back in Canada now.
If we don't believe in God and his sovereignty, we are doomed to frustration because we will replay our "bad decisions" over and over again. But if we believe in God and his sovereignty, at some point we have to move on. The only way to do that is to focus on God and his greatness and his goodness. Romans 8:28 has to come into play at some point. Presumably the "all things" must include even the worst things that happen to us and the foolish things we do. It must include even those decisions we think we would make differently.
When we factor God in, then and only then can we let go of the past and say, "Lord, I believe you can use those things for my good and your glory. I believe it because your Word says it, and I also believe that if I trust you, you will prove yourself faithful to me even though right now I have my doubts about how my life is working out."
We can't change the past, and even if we could, we can't always be sure we would improve things. But we can trust the Lord, go forward, do our best, and leave the results in his hands.