Laura,
Since you have confirmed that you are not pregnant the other causes for a missed period can be due to impaired hormone levels. This can occur normally because of physical or emotional changes in your everyday life. Stresses of relationships, new job, moving, school, heavy exercising, illness, weddings…and so on can cause an interruption in your cycles even if you are about to ovulate. Just the stress evident by taking six pregnancy tests is probably causing your body to delay ovulation!
According to the Billings Ovulation Method (BOM) your charting would be able to help you notice this interruption. By learning to chart the basic infertile pattern and the fertile days a couple can achieve or avoid pregnancy by choosing the days for intercourse. I agree, you both have been taking a risk with such an important life-changing event. Please see a BOM teacher and get instruction on charting so that unusual patterns can be detected.
Also you and your boyfriend can take this time to let your relationship mature and decide if you will be committed as a family. The other option would be to start over and let the emotional relationship develop without the worry of becoming parents before you are ready!
Angie Frausto