Yes. In a word yes I have had doubts from time to time. It's like when you are in the middle of something, the coin of chance is flipping before your eyes and you are trying to will it to land heads up or something. Faith is believing that no matter how that coin lands, God has handed down his will through it all though we don't know how or why. The voice of doubt is always nagging us to give up precious promises given to us freely by God. So it is a struggle to hold onto the promises of God.
"Like a ship returned from a voyage I feel,
lost and wearied beyond appeal,
I wonder aloud if my feet will find,
the home that leads to my rest of mind,"
Wow its nice when you find that place of rest, but it seems so short in hindsight when your are back in the whirl of living.
As far as defending your faith goes, it takes numbers to win at anything. Imagine if you showed up to a football game and only 2 players from one side showed up while the other team has 65 people. Whose going to win you think? I keep my faith quietly with me. I always have a little Bible with me wherever I go, the people who know me know I carry it, they know where I stand and they respect it by and large. They come to me from time to time with morality questions, and I think that is the key. When someone wants to know the truth they will come find you, rather than mock you from a crowd. And let me emphasize, I am not saying you need numbers to win, I am saying don't pick a fight with a huge group of non believers. That, unless directed by God, is just going to alienate them and cause a lot of ruckass usually. People come to God out of free will when and if they are ready (usually in what could of been there final moments), they will call out to Him though they don't know him.
Don't be afraid to say you don't know when debating theology. Every has a mind, and a perspective. You can go nuts trying to justify yourself to others honestly. If they haven't accepted Christ, honestly they are for the most part incapable of seeing the revelations of God. So find people who are with you in Spirit, if you want to see the people you are around come to the Lord, pray for them every night. Get a group, can't stress that enough. But just remember when you start battling for someone soul to be redeemed there will be a fight, so put on the full armor of God always and live right if you are going to do that and if you have the commision from God in hand.