Why do I get blood and burning sensation while passing hard stools?

Hello,

I am a 23-year-old female and had a flexible sigmoidoscopy just a month ago because a little bright red blood was coming from my rectal area. I have a burning sensation when my stool is really hard, which is a problem in my case. When the doctor put the camera on during the sigmoidoscopy, he could not see any anal fissure, but there was a tiny hemorrhoid. But he said it was not the cause of the bleeding. The doctor saw a very small reddish area in my rectum, but it was very mild red. So he took a biopsy from there in case it could be the start of ulcerative colitis. Test results came, and he said that there was nothing wrong. Enemas also may cause that mild redness in that area. He gave me laxatives and told me to consume fiber-rich foods. So my questions are how quick anal fissures can heal themselves? Are they really like lip cracks when they are damaged a little? My bum really burns while passing stool, especially when I am constipated. But sometimes, when I wipe, there is a yellowish fluid that is not diarrhea. Is it mucus in my stools? Or is it because of anal fissures? My health anxiety is also not helping my case, although the doctor told me that there is nothing to be worried about. I would appreciate it if you gave me answers in detail. Thank you.