I know this is making you worry and fret like crazy but truly you cannot possibly get pregnant in the way you describe. The chance that there was sperm on the toilet seat is very low. The chance that it could survive in that environment for any length of time at all is virtually 0. And beyond that, you washed your hands a bunch of times and then went out into the elements before arriving at home (again another environment that sperm would not survive).
I am glad you let us know a bit more about talking with your mother, doctor, etc. If you do not like this doctor then at some point you should find one you do like. If I have a choice I always choose women doctors. They are just better at explaining things and at communication in general in my experience. Of course there are exceptions but maybe you would feel more comfortable with a woman? You don't need to do that right now but everyone should have a doctor that they like and are comfortable with. Sounds like you don't feel that way about the one you went to although it does seem like what he told you is very good advice. Of course he had no reason to give you a pregnancy test and I don't think you need one but if you feel better getting one, then I would find a free clinic, planned parenthood or someplace where you can get one just to fully put your mind at ease. I think you know deep down that you cannot get pregnant the way you describe because you know that science tells you it isn't possible.
If you think talking to the psychologist that the doctor referred you to would help, then maybe you should make an appointment and go see them. I don't know anyone who hasn't needed that kind of advice at some point in their life so there is certainly no reason not to go and talk with a psychologist at least a couple of times. It sounds like your anxiety is getting the best of you. You said you are a senior in high school and being near the end of high school can be an awfully stressful time because a lot of changes are taking place–both in your body and in your life. So don't be so hard on yourself.
Have you ever done yoga or any type of meditation? I think some of the other mentors suggested doing some exercise or relaxation techniques or just maybe getting lost in an activity you enjoy. But the one thing I would not be worried about given what you have described is being pregnant. It just does not happen that way and there will be a point when you are more relaxed when you will look back at how you are feeling now and realize how stress and anxiety can play tricks on your mind and make you think illogically. We have all had it happen to us which is why I know this is true.
Are you looking forward to graduation? What are your plan? I am much much older than you but I am really envious as that is such a wonderful time in one's life. I know it may not seem like it because it is also a bit of an uncertain time, but you have so much to look forward to. Keep letting us know how you are doing but take some time this weekend to try to let go of some of your stress. Hopefully you will stay in touch with us.