Be chasing (one's) (own) tail American British verb phrase
Used when someone is busy to do a lot of things but achieve little and do not lead to progress
I have been chasing my tail all day, but I haven't finished any tasks.
Used when one wishes to engage in sex with someone
They admitted that they were chasing tail in the bar last night.
The verb "be" should be conjugated according to its tense.
If you fall for someone's trick the first time then they are at fault.
Are you going to mess with me again? Fool me once, shame on you; fool me twice, shame on me.