On finding confidence through a brain addled with uncertainty

I'm a professional pessimist. While on autopilot, my brain will easily conclude that I will not get the job I want, the guy I like will not like me back, and my roommates will always secretly hate me. It's a constant battle in my brain--the natural tendency versus my will attempting to override the overwhelming desire to give in. Once we reach a certain age, I feel that we all have at least some of this in each of our heads--this is the difference between those who succeed in what they want and those who settle with what comes to them.
Pulling from personal experience, which is the best thing one can do in this life, I'm thinking specifically on my inner battle during my engagement--I kept thinking I was settling, but then I'd tell myself that I loved him...but then I'd turn around and know that I was settling (sorry, if you're reading this, Robert). This battle was what, in the end, made the decision on whether I was going to break off the engagement: I felt like I needed more in a relationship, therefore I was settling in this one because it was secure. Security is nice, but it isn't nice when it's not completely set in your mind.
A more positive experience on this side of life: getting a job. I have a theory that once I get an interview, it hardly matters what I say and what accomplishments I have, it completely depends on how I hold myself and how I talk about myself and my past experiences. I used to go into interviews without thinking about how to act, and I would end up acting incredibly insecure, not at all proud or confident in myself. During the interviewing process of getting the job I have now, though, I made sure I beamed like those were the best days of my life, and I talked about myself like I was special and important--which I am, to me. If you're not important to you, who are you ever going to be important to?
And what does it mean to be important to someone? How do you act toward that person? You show respect, you give compliments, you smile at them, you wish you were them. So, why not treat yourself like that person? You only have one life; why go through it feeling like you're the scum of the earth? That won't work. Confidence starts in your brain before you're able to reflect it in your appearance and mannerisms.
Let me spell it out: treat yourself like the most important person in your life, and you will realize your worth. Realizing your worth will build the type of morale that will give you the confidence to believe that you can succeed in this life--even if it's success in the smallest of battles. Don't ever think that you're not worth it because you're always worth it, even if it's just to yourself for a little while.
And I'm not saying I'm perfect at this. As I said at the beginning of this blog: it's a constant uphill struggle, especially for anyone who's ever dealt with depression or anxiety (I'm guilty of both). The thing that counts is that you're trying--you're always trying with hope in your heart that things will turn around.