(Converting comments to answer since people always yell at me for that)
You clearly came off as intelligent in the interview and capable of learning something given enough time. Possibly this 'sitecore' project is something that needs work to be done on it, but with no definite deadlines, so its more appropriate for someone who is still learning due to the lack of deadline, then once you have ramped up and are capable of this "advanced .NET", then they will have confidence that you can work on other things that have stricter deadlines. (all just possibilities)
Quite honestly, companies know what they are getting when they ask for a junior/entry level position. I've worked at 3 different places and it ALWAYS took me 1-2 months to start feeling comfortable and then another month or so to know a good portion of the system. If they are pressuring you more than is physically possible for you within 40 hours a week (or whatever your schedule is without overtime), then simply talk to your manager and let him know you need time to ramp up, you don't want to wait until you miss all your deadlines.
Confidence comes as you gain experience. In a month or two you will realize that your not half bad and actually just as good as many people who have been there for a long time. Forget your background and stop comparing your education to others, simply focus on the work you have to do and admire the work you have done;nothing good comes from comparing your education levels.