How can I up my confidence level in my technical skills? Let me paint a picture of my background in brief. I did my undergraduate degree in mechanical engineering. I was a good student (almost all A's) till my high school. However, I struggled a lot in my first two years of engineering. I failed 3 subjects and had to retake them. I did fairly well in my last two years and managed a GPA of 3.7. But, my early failure played a heavy toll on my self confidence. I always felt and still feel like I am not worthy for my grades or getting a job.

I was interested in research, so went to a better school for master's degree. Again, though I had very good grades there, I struggled a lot during my job interviews. I always felt inadequate and unconfident about my ability to sustain in that job. As a result I did not get selected in those companies. I got offer in a small company which hired me only looking at my grades. My professors also said that I deserve better workplace but I didn't get it only because of my lack of self confidence and technical speaking skills.

I figured that, since I loved doing research, I should go for a PhD. I worked hard during the last five years. I tried coming out of my comfort zone, try different things, work on challenging projects. I started feeling confident midway through my PhD. However, now I am in my last year and towards the end of writing my thesis. And I am feeling the same anxiety of technical inadequacy and unworthiness.

I am again getting the feeling that I am upto no good and I can't get a decent job or postdoc position with my skillset. Only issue is that I am 29 years old and I will have to get a proper job soon to support my family. I don't know if I am capable of doing that. 

My question is, assuming that I suffer from severe case of imposter syndrome, what's my remedy? How can I improve my confidence levels during an interview? I want to feel alive during job search and interviews. I don't want to feel all hopeless and inadequate during any future opportunities. 

Are there any suggestions?
My advice is to focus less on the abstract unknowable questions "how good am I?", or "am I technically skilled?" - and instead focus on the concrete things which you've actually done. Focus on the achievements you made in your education. If you like, make a list of all the problems you solved and things you did in your PhD. When you get asked questions in your interviews, don't think about whether you are "good", just think about all the things you did and (metaphorically) give the interviewer that list, and let them judge for themselves if you are a good fit. The other clear solution to having nerves during interviews is simply to do a lot of interviews. Try to schedule some "easy" ones early, so that when you get an interview with a company you really want to work with, you have a bit of practice under your belt. You can also of course have mock practices with your colleagues and friends.