Urgency for a bowel movement constantly but when i go sit down to have a movement it doesn't always happen. also have white mucus stuff in poop.cause?

Need a rectal look: This could all be from constipation and stool in the rectum leading to a feeling of needing to go - but with the stool so hard it doesn't pass easily (and mucous is the body's natural lubricant in this situation). But you could also have rectal irritation (proctitis) and a direct look at the rectum (sigmoidoscopy) is the only way to separate the 2 possibilities.