Ultrasounds done in the 19th week, 31st week or 33rd week cannot tell reliably when the baby was conceived, since fetuses can vary in size by then. (By the 40th week, its size can vary by plus or minus up to 3 weeks from average.) If you are going to try to use an ultrasound to figure out when conception might have been, you need to do one in your earliest weeks of pregnancy. By week 19, the size of the baby can be ten days off the average (larger or smaller).
It's also not reliable to use when your last period was to try to tell when you ovulated, because women differ in when they ovulate in relation to their periods. (If your periods always come totally like clockwork every 28 days and never fail, you might be able to assume you ovulated at 14 days before when the next one was due, but ovulation is not triggered by the last period, it comes when it wants and then two weeks later comes the next period.)
All you can do is either spend the big bucks and get a prenatal DNA test (with both possible fathers) now, or get a regular post-natal DNA test (with both possible fathers) after the baby is born, which latter test costs only about a tenth as much as a prenatal test. Given how close you are to 40 weeks, unless you are quite flush financially, there doesn't seem to be a lot of point in spending hugely for the prenatal test, but if you totally have to know for sure now and don't think you will make it through the next 6 weeks without knowing, use Ravgen or the DDC. (Don't fall for those Internet cheapie labs.) Be sure to test with both guys, whether you test before the baby comes or after, because one guy's test will confirm the other's. (One will get a negative and the other will get a positive, removing any later worry that the test was wrong.) If the guy who doesn't want to have anything to do with this gives you flak about taking a test, after the baby comes you can legally enforce it. Since you only have 7 weeks or so left, you might consider talking to a lawyer about what it would take to get the guy to test if you think this is going to be a problem. It will make you feel on top of things to know you have authority behind you in the situation.
Also, for heaven's sake, stop moping. There are women (read the Infertility forum if you don't believe me) who would change places with you this very moment even given that you don't know for sure. You are going to have a wonderful baby, and you have at least one nice boyfriend. Your life will go on and will be happy, just move forward with it.