The problem as i see it here is that we know that your boyfriend know about the jealousy you experienced, but what we don't know is how it came out. Did you tell him? If so how did it come out? Did you sit down with him and calmly share how you felt and why? Did you accuse him of anything? Did you get angry? Or did he find out in some other way? How did your jealousy affect how you treated him? See, there's lots of possibilities that can make a difference in how much sense his reaction makes.
Also how long has this relationship been going on? Have you been dating a few months? A year? Years? This is another factor that could determine what kind of advice to give. If the relationship is only a few months old then it may not be worth it to try to fix.
How long has he and this other woman been friends? The longer their friendship the more likely it is you will have to accept it, and her. If you aren't capable, then that's going to be an issue. Dating someone who has close friendships with people of the opposite sex is tricky.
Now lets say you did have a calm discussion without any accusations, then there are two reasons for his behavior that are possible.
One is that he values trust to a high degree and if you show a lack of trust in him it may affect him stronger that it might others. This could possibly due to how he was raised, or a situation from his past or just something natural within him.
Or he is insecure and that any sign of a lack of trust makes him feel as though he's wrong, and this makes him feel lower about himself. Though i find this one the less likely of the two, given how you have described the situation so far.
The other reason is that he has something to hide. Often times people who have something to hide when they are accused will have their conscience pricked, and rather than own up to it, they try to take the focus off themselves, and can even use the situation as leverage against the accuser. Make them feel guilty or that they need to make up for what happened.
If your feelings came out in a different manner, accusations, anger, etc... then he will be justified in his feelings.
For example, i tend to have a lot of female friends, which, of course means i have close female friends. If i were dating a woman who came to me and said 'hey, i know you and X woman are close and this bothers me for X reason' and she was calm and rational, then i'd be fine and willing to discuss it.
However if she approached me in an emotional state, or hurling accusations or things like that, then yeah, i'm going to be mad and probably hurt as well. And when you drive a wedge between yourself and who you're with by doing such things it can be very hard to amend. Even married couples together 20 years will have problems if there is an accusation or suspicion of cheating, even after it's cleared up. Because the accusation does damage, doubly so if it's false.
Now that you've already expressed a lack of trust, you seem to want things to suddenly go back as if nothing happened. And that when you say you trust him he should just accept that an believe it. Well, he's been believing that for as long, or longer, than you've been dating, and now he's found out that's not the case.
So, all in all there is a lot of information needed, but not given, to accurately make any sort of real advice or suggestions. I tried to cover the ones i could think of. But bear in mind, all advice given so far is given without having all the facts, so it may not be the best advice in relation to the whole story.
