sweetie, don't ever be afraid to say to someone who comes to reception that you are new, you don't know the answer, but you will find out - or if there is nobody to ask, to do your best. It is not a failure to admit that you don't know something - rather, it's grasping the situation.
Maybe you need to try to get to the bottom of what caused your low self esteem, so that you can come to grips with that and realise that in fact you ARE good - you got the job, so you can't be that rubbish! - and stop using horrid negative expressions like "I suck so bad" - no, you don't, you're just learning. You must start to believe in yourself - if you think you're cr@p and tell people you're cr@p, you'll come to believe it, and you're not.
It isn't a shaming thing at all to have to ask your manager for help - I did it today about an unusual situation which cropped up, and I'm confident in my job usually - and then I found that she didn't know the answer either, and had to ask her line manager what should be done. That's not failure, it's a learning curve.