Decisions in general can be classified into following three
1- Clear: There can be decisions that are pretty "clear and easy to take" as well as "clear decisions but hard to take". e.g. Quiting smoking is an easy decision but its hard to take it.
2- Can be clarified if serious thought given: There can be decisions that are unclear to start with, but if one gives a serious thought then right and wrong becomes clear. Ways to give serious thought might involve consulting existing knowledge on that specific issue, or taking a pen and paper and start writing all the variables that are expected to be affected. Giving each factor a weightage and evaluating. These sort of situations are common in management and techniques like SWOT analysis or similar other can be used.
3- Unclear: Some situations remain quite unclear even after deep thought. In that case, one can follow his gut feeling but one should try to do his level analysis before that. I prefer taking the decision based on whatever analysis+gut feeling combined points to and then do the istakhara for that choice. (What I have inferred from the wordings of Istakhara dua is different from common understanding. I think one decides first and then does the dua for istakhara. I really appreciated each word of it when I had to take some decisions that I was not clear of)
Whatever decision we make mostly there is no guarantee of it to be correct or not. Generally, time defines whether the decision was correct or not. We don't know how dots will connect to make the picture in future. I can think of many things that I did not consider good for me and eventually turned out good for me and vice versa. After one takes decision, he should not bother too much in terms of repenting even if it turns out to be wrong. But one should seriously evaluate where it got wrong and try to correct it for future. Thats what learning is, making mistakes is part of learning. Even prophets AS can make wrong choices (Like Adam AS) but its the learning and correcting that matters. Meanwhile, we should keep in mind what is in our control and what is not. Making choices is in our hand, outcomes are not. Outcomes are related to effort, but not guaranteed. Allah has his own plans and absolute success and failure is in the afterlife.