Can anyone help me understand antibiotic resistance?

Hi Guys,
As you know I’ve been complaining about antibiotics. Well yesterday was my last day, after getting blood tests done the Dr called me back in, asked about my symptoms, told me my blood tests were perfect and that I could stop. She said the idea now is that a short course is better than longer and to allow the body to mop up the excess bacteria. It was my uneducated understanding that excess bacteria if left over is what gathers the resistance? However according to my Dr longer courses are not considered more dangerous than shorter. So then my next question, I bit my tongue two days ago, so I assume that the antibiotics I’m taking will have an effect on that as well in that no bacteria will get a chance to multiple, however, now I’m off antibiotics today so that does mean I’m inadvertently causing a mutation in the possible bacteria in my tongue now? I know tongues heal fairly fast and it’s a really just a curious question. My health anxiety needs help I know this, but I just feel perhaps if I understand this a bit better it might help me take future courses of antibiotics without this ridiculous stress!