As a practical matter, since you are at 26 weeks, it sounds like an abortion is not being considered, so that gives you some time to sort it out, correct? What would you do if the baby came from the sex with the other guy? I'd put together a plan in my mind for either scenario. It will help you stop freaking out.
Regarding some statements you made in your story -- October 19 is not necessarily "the exact date of my conception date!" as you put it. If you were to have had an ultrasound in your sixth or seventh week, the estimated due date is more likely to be right or very close, but an ultrasound any later than that will have a margin for error that gets bigger the later in pregnancy you have the ultrasound. By your 12th week, the margin for error is +/- 7 days. By the 40th week, it is +/- 3 weeks! Unless your ultrasound where the tech told you July 12 was very early in the pregnancy, you can't be certain July 12 is your exact due date, which means you can't be certain October 19 is the day you got pregnant.
And of course, if you had sex with your boyfriend on the 18th and the 20th (this set of dates certainly seems to bookend the world's shortest breakup), his sperm will be in there as active players in the race to the egg even if you did ovulate on the 19th.
Incidentally, you said "I don't think he understands that the 26 weeks starts at the last day of LMP." Good thing he doesn't think that because it starts at the first day of the LMP, or a calculated first day if the woman's cycles are irregular. (I don't think that throws off your calculations, though, you must have been using a conception calculator? because if the tech was right in saying July 12, you did get to the correct assumption that the estimated conception date was October 19.)
What I think I would do in your shoes is three things. (Unless you have the huge bucks to do a prenatal DNA test, then that would be the fourth thing. Or, actually, the first thing. Go with Ravgen or the DDC if so, and test with both guys.)
The three things are:
1. Keep your head up. Your boyfriend had a lot more chances to fill you with sperm (I assume you are saying the sex with him was unprotected?) than the other guy.
2. Don't let the dates scare you if your ultrasound was later in your pregnancy than your 7th week.
3. Realize your life will go on and be successful and you will be a happy mommy no matter what the paternity test ultimately reveals.