When we follow our vocation we should do so with all our heart, all our mind and all our will just the way Jesus tells us to love God. Notice he doesn't say simply with our heart, but also with our mind and will. This is important. We need to use our mind so as to make sure as far as we can that we are not making a mistake; however, we are not talking about "cold rationalism", our independent mind as sole judge and jury, but using our mind enlightened by faith. And Jesus also tells us to use our will because to follow Christ we have to "freely impose his will on our life" and not obey only our feelings. Grace, of course, acts on our will to move and strengthen it.  Now, our doubts and second thoughts can be due to many reasons, but very commonly they have their root in the fact that the vocation is so intangible, so we have to put them in their place. In other words, we don't have to have absolute guarantees and the answer to everything, but we do need to make sure that what we are doing is not unreasonable; we have to know the reasons for our actions, and know the reasons for our trust.

I think you have to ask yourself, and discuss with your spiritual director the reasons behind your particular uncertain feelings regarding if this is the way you should go. They may be very vague and just normal fears, in which case you will have to overcome them with your hope and love, and follow through with the vocations director. The re is also the outside chance that they reflect the promptings of the Holy Spirit who is leading you in another direction, perhaps toward priesthood in a religious order, for example , or toward the lay life, although from what you say your doubts are simply where you should pursue the priesthood.

Try to examine the reason for your uneasiness. If you discover it comes from your inclination toward community life, an attraction to a particular type of ministry, or a particular spirituality, the example of a particular priest you have met, or a book you have read, etc. , it could be indicative of a religious vocation.  A logical next step in that case is to look into whatever religious orders you have heard about and which attract you, or if you have none in mind at this stage do a little research on the web and see what touches your soul.

I have also met young men who have a real diocesan vocation but are concerned because they feel their own weakness and wonder if they need a community, or they wonder about the seminary they will be sent to , or are bothered by some particular local situation. If anything like this is behind your hesitation the best, and really the only way to solve it is by being very upfront with your vocations director.

God bless.