Don't ever say your not good or smart enough. That right there causes low self esteem I know. Its alright that you don't know all of what is going on in class now, you've just started and professors do take that into consideration. Also don't compare yourself too others, I have a nasty habit of that as well and it gives me nothing but anxiety and depression because of it. Stay positive, study, and you'll do well and get the job your trying to get!