To my knowledge, there has never been a documented case of an STD
being transmitted through a toilet seat. One would have to be extremely unlucky for that to happen. For starters, some of the common STDs such as chlamydia and gonorrhea would not be easily transmittable from a toilet seat. This is because the bacteria would need to be transmitted through a media (the plastic seat) into your vagina or penis. I cannot see how that would be easily accomplished. The herpes virus does not live long outside of the human host, so I don't think the toilet seat could accommodate this. The risk of getting HIV from something like this is near zero.
I understand your concern and thus, I think it would be best if you scheduled an appointment with your doctors
">primary care physician. The two of you can discuss the best methods of reducing your likelihood of getting an STD. In addition, you can discuss good public restroom hygiene which is never a bad idea. Primary care doctors can also perform routine STD testing for gonorrhea, chlamydia, syphilis and HIV. You can and you should at least have the first two of these done yearly if you are sexually active. Good luck.