Go to a Dr. & get a true test...the drugstore kind are often wrong. Don't know where you heard about sperm living only 30 seconds! If that were true none of us would be here.
If you keep stressing yourself out to get pregnant that in itself can defeat your objective. Relax...it will happen if & when it is meant to. You didn't mention your age but from the 'sound' of your post you are quite young... I think sometimes that can have an effect too. What's your hurry?
You could try : after sex put a cushion/pillow under your bottom to raise it up into the air. This helps the sperm swim to the egg by gravity. Stay that way for at leat 30 minutes.