There is no real risk here.
STDs like gonorrhea and chlamydia live in the throat, so when someone uses their mouth to put the condom on, even if it's not going on the right way, it's not collecting the bacteria.
STDs like herpes and syphilis require direct skin to skin contact to transmit, so even if a bit of the pathogen was on the condom, it wouldn't matter.
The skin on your scrotum is too thick for pathogens to penetrate, and it's a very unusual place for syphilis or herpes infections.
I don't know if your guilt comes from going outside a relationship, or from using the services of sex workers, or maybe both, but don't confuse guilt with risk.