My motto is: When in doubt, let it go. Not saying the shot has to be perfect, but you have to be confident that it's a shot you can make reasonably well. I probably pass on more shots than I should, but having made a dubious shot once a few years ago, I've learned that it's better to regret not taking a shot than to regret taking a bad shot. Each hunter has to make this judgement on his own, but I personally believe it has a lot to do with one's own definition of ethics.

Don't beat yourself up over it. Only take the shots you're comfortable with. As you gain experience, I think you'll find that the assortment of shots you feel comfortable with will expand.