Q: How do you get your boyfriend to trust you after you have deceived him twice?
A: Well, it's real simple: you don't. It sucks, I know, but that's life.
Of course, I know you are reading this with a "gee thanks, doc" response, so I'll continue. Because this person is still your boyfriend, I can assume that you both probably want to work it out. Good for you, I wish you the best. Unfortunately, human nature may work against you. See, one of the instincts that we have programmed into our heads is the tendency to place greater importance on bad memories than good ones. If you think about this in another context, you'll see how it makes sense.
Suppose an early human stumbled onto 3 types of berries and ate one of each. The first was bland and had no taste. The second tasted awesome, full of flavor and loaded with sugar. But the third was so bitter that it left a fowl aftertaste and even caused the poor guy to spit it out, maybe even throw-up. Which of the three berries are going to leave him with a stronger memory? Good taste is often associated with nutritional value but bad taste is usually associated with poison (convenient how that works). So even though it may be a good idea to remember which berry tasted the best for the next time you're hungry, you better damn sure remember which one may kill you.
Our selective memories help keep us alive and from repeating mistakes. So let me get back to your boyfriend: I'm sure that 99.9999 percent of the time that he's known you, you have not betrayed his trust. Hell, there may have been times that you've proven your trustworthiness. But, and this is a J-Lo sized but, he will always remember those two times that you deceived him. I'm sure that every time you fight now, he brings them up. I'm sure it affects the way he acts with you in certain situations, and I'm sure that no matter what you do now, he refuses to let it go. Right?
This isn't fair to either of you, and if the issue persists then I doubt that the relationship is worth the trouble. However, another cool thing about human nature is that we are adaptable and have the ability to change or overcome our instincts (just watch "Fear Factor" for instance). So it is possible that he can learn to trust you again, although trying to get him to understand on a rational level and act on an emotional one are very difficult tasks. His awareness of what he is doing is key, and he must try to keep in mind that you've only deceived him for a fraction of the time you've known him despite how intrusive those two occasions may be. But the bottom line is this: if he is going to continue to treat you like a poison berry, then he shouldn't keep sampling the fruit.