My back tenses up, pelvic floor feels stressed, muscles around anus gets sore until i pass stool that's in my rectum. weak pfm. how to alleviate this?

Soften stool/diet,,,: Likely the best way to stop this is by getting more fruits and vegetables in your diet. This way your stools may be softened and so there'd be less pain with bowel movements. The stress would also likely be reduced. If there is still pain/stress with movements, you might want to see your doctor for an exam and recommendation for otc stool softeners or get a prescription for one. Good luck.

Change diet: There is high likelihood that you are passing very hard stool. You need to find the way how to make stool softer. Do not put "all eggs" into so named stool softeners even they are indeed help a little. Major here is what and how you eat. The first step is to start eat soups, broths every day. You nay also seek dietitian consult. What you need is to increase water content in your food.