Glue stains appeared on veneer after first coat of poly. Can I sand it down again to erase them? Or should I just give up on woodworking/ life altogether?

Don't give up, try a scrapper if the finish sits on top of the vaneer. If the finish soaks into the vaneer and you don't want to sand through, you can always sand back a bit and then use touch up markers with a fine tip using a close match to the wood grain colors shades to mimic the wood grain pattern overtop of the stained sections. Then reapply finish. Or just live with it as others have said. You are hyper critical of your work but others who admire and don't know about the "mistakes" likely wouldn't notice and/or care.