Sounds to me like your anxiety levels are rising and youre focusing them all onto one point, the pimple.
I had something similar when I was younger. I had my nipple pierced and it was a botched job. I used to check my breasts multiple times a day to check they were ok. This went on for months. I was forever taking toilet breaks at work or hiding in a cupboard to check them throughout the day.
Now you may not be checking them as much as I was, but you'll end up down that route if you dont stop now.
Try to cut back the times when you check it. Maybe give yourself time in the morning and before bed to do this. And do not check it in between these times.
You should also impliment a worry time. Decide on a time of day, that you can just sit down and worry. if you try to worry at any other point, tell yourself you will deal with it at worry time. This will hopefully stop you worrying yourself all day every day.
I hope this helps.
Just remember you will be ok, your GP isnt worried and nor should you be. The lump wont change in the space of a day, so if anythign did change you wouldnt know about it before the lump did!