I NEED HELP

I need help …. I am 1/3 into a vest / cardigan and realize I am knitting one size too big… anyone else done this ? Any suggestions?

Welcome to the forum!
What is the name of your pattern and designer? Are you about a third into knitting the back or is this knit in one piece?
Painful as it is, the best solution is likely going to be ripping back and starting again. Sometimes gauge swatches to determine stitch and row gauges lie. When we get to knitting the project the gauge is off. Use the knitting you have now to measure your stitch and row gauges, and compare those numbers to the pattern gauges.
Let us know the pattern however since that may help with figuring out what to do.

Have you measured it to be sure it’s too big? I often make a different size than I think I’ll need, based on gauge and actual outcome.