Here's the thing: People's dissertation work is usually not an earth-shattering achievement.
I feel that I have just not done enough.
I feel my Ph.D. was sorta-kinda enough; and a "strict version of me" might not have accepted it.
(in hindsight I developed a better opinion of it. I now feel it was solid enough work.)
My work is simple and I also feel that I have poor theoretical foundations
Well, your work may be simple (or it may not), but most research findings are simple once you have all the context in mind. The point is that you discovered or invented something new that wasn't known / didn't exist before. You have done that; and that's basically why you deserve your Ph.D.
As for the lack of theoretical foundations - ugh, tell me about it! Even at my peak I felt like an empty-headed fool compared to the "sages" of my field. Again, it's possible that you're under-estimating yourself, but it's also possible that you're setting unreasonable expectations: Most Ph.D.s in your field know less than you do about what you've been studying.
and am going to be an embarrassment when quizzed by my committee.
From best to worst:
- You'll probably know enough about what they ask you to say something non-embarrassing.
- "Esteemed opponent, that is an interesting idea. I have not considered that avenue during my doctoral work, and would need some time to consider it. It may well lead to further results beyond my own work."
- "Esteemed opponent, I am not versed in [insert complicated subject here]. It is possible this field may have bearing on my findings."
Now, is no. 3 embarrassing? Well, it might be. But it's also the truth. Don't try to run from it in your head. You know what you know, you've done what you've done, you are who you are. That's not shameful.
I am not feeling confident about presenting my research
You should present what you did. Don't try to glorify it, nor to downplay it. What the question/goal/challenge was, how you approached it, how/why it worked, what the results were, what the consequences/corollaries/implications are. You can be "confident" about that - since these are just facts.
I am not theoretically strong enough for doing my PhD on my research field
You are strong enough in the sense that you've already done your Ph.D. work. You're not auditioning for the title of all-knowledgeable theorist.
I am not able to push out the feeling of guilt and the impending doom. Any advice on how to tackle this?
As for the guilt - don't try to make it disappear. Just try to separate the presentation from the guilt. You present the stuff that you're not guilty about.
As for the sense of doom - I tried to make the non-doom'ishness more palpable for you; I hope this works.