Dear Adrian,  

Your question opens up a facet of vocation that may not be easy for me to explain or for you to follow, but I'll try.

You ask about past faults and if someone might possibly be turned away from a vocation because of them 'even if they had a true vocation and had been living a moral life for the past couple of years'. It is helpful to dwell on this, because it is the same as asking, 'can I be turned away even if I have a vocation?

Here is the essence of vocation: it is a call. We don't choose, we are chosen. Even when we think we feel a vocation we have to find out if God is really calling us, if what we feel is true. God chooses, we only answer.

Serious sin in the past can affect a vocation. It can in some instances be an impediment, and it might indicate clearly that a person is not being called by God to religious or priestly life. I have no idea of the nature of the failings you are thinking of, and to find out if what is bothering you is an impediment you would have to speak personally (do so in confession if you want) with a priest you trust, and accept his advice.

You may find out that what you did, or its effects, is an impediment, and yet still yearn to be a priest. This is where your faith has to come in and you have to understand that even though you are generous enough and would like to serve God as a priest, he is not actually calling you.

Granted, this is sometimes hard to accept. We tend to think the vocation consists in me wanting it, but that's not the case. It is God calling me. Sometimes we don't feel like it and he still calls, other times we want it and he doesn't. In one case and the other we have to trust him, and accept what he says.