Doubting something may imply that you do believe it, but have reservations; or that you've been convinced at some level that you believe it, but are finding a realization coming up from deeper inside you that in fact you don't believe it.
Specific reasons for doubt may not always be identifiable, although sometimes a fear that to express the doubt in words would be blasphemy. Reasons for doubt may include there being an apparent contradiction implied by the belief, either against another belief, or against ``common sense'', or against your more general observations.
My approach to this, as to other topics, is to dig down to the fundamentals, asking ``What is implied by this belief?'', and answer the doubt at the important levels, not in the surface details where it may first be perceived.
Here I list particular beliefs that I think are sometimes doubted; but I don't think it's appropriate for me to try to answer the doubts, but simply to point out ideas that may be helpful in working through them for yourself.
There are some things which are commonly held as Christian beliefs but are not truly part of our faith, and may in some cases contradict it. These are typically humanistic or egocentric. An example of a common belief that I no longer consider necessarily true is that God's love for us includes Him willing us to be what we see as ``happy''. His purpose for us is much greater than that, and our giving ourselves whole-heartedly to following Christ entails our being prepared to give up pleasures of this life for His sake.
Some time ago, I working through a book of meditations (Love, a Guide to Prayer by Bergmann and Schwann) which has for each day a scripture reading, a devotional commentary, and a point to think and pray (meditate) on. One of the readings was Abraham being asked to sacrifice Isaac [:], and the question was something like ``Identify your own personal Isaacs: What do you think you would give up only reluctantly were God to ask you to?'' . At first I thought of my friendships... that the hardest thing would be to be alone (such as becoming a hermit or recluse), but on further thought it seemed to be my faith that would be the worst thing of which to let go... and I wondered ``What if God were to ask me to give up my faith in him?'' and the question seemed to be important, not just hypothetical... as if he might ask such a thing of me.
I puzzled greatly over this and next time I was on retreat, asked a friar about it. He simply said ``It's not your faith to give up anyway. It's God's faith'' and that gave me an important key... in the supportive environment of the friary, I was able to let go of, even to reject, what I had seen as God... to let that internal image fall away because I knew that it was only an internal image, and find it replaced by a new contact with God, with less of an image to it. This is an exercise I must repeat from time to time; and it is one that I found all-important myself, so I've written it down here in the hope that it may be helpful to others.