If you really want to develop a backbone, do some pushups. Don't waste your time searching for excuses.

I found this book, very useful. To some extent, it helped me to
shape my character. I feel stronger now, while dealing with tricky issues, both at work and with friends. Try to finish the whole book and be patient. You should try to use some of the techniques in real life, to see any change. You will have a chance to understand that being humble, all the times, doesn't necessarily make you are a nice person. You should respect yourself first, before expecting some one would respect you.

this is one of the best book of my life.I read this along with "How to Win friends and influence people" by D. Carnegie and felt great.It is really really a great book, very practical.
If u can practice the exercises adviced here (ie, going to a shop and ask for a change without buying anything), you would surely become a confident person.

best luck!
buy it!