hi cris
I am not sure what it is you think you need to confess but whatever it is, the boss you say was there at the time you omitted to scan some items thru will i assume have been concerned, quite rightly, that these items may have been a loss to the business.
Without being too harsh, in retail it is not easy to distinguish between carelessness and dishonesty, if there are several incidents of stock loss one explanation would be the staff are failing to process the transaction correctly at POS.
From my own experience, well trained stafff who for the most part perform well under pressure, should not be making simple mistakes like that anyway, if you feel that the Boss may now be watching you more closely now then that is understandable but it is the boss's job to be aware of any employees weak spots, if it really bothers you then maybe if you ask him to review the staffing levels during busier times, or maybe introduce a customer service window during the day when it is really busy, to ensure that noone is put under pressure and all staff are focused just on the customer and shop floor. A no rask zone between 12 and 3 on a saturday for instance would help solve the problem of having to leave the till point yourself during a transaction.
Not every store can be so professional i guess, but there should be a different approach, and certainly when you are so busy you are performing all the duties yourself as you describe, the boss should not be simply watching what you are doing, they should be actively supporting the process.