Oh dear

copperhead .. it's human nature to respond to other's according to our emotions and how we are feeling at the moment, moreso than what a person actually does. Additionally, our views of others are slanted according to our emotional well-being at any given moment. So, with this particular situation, I'm gonna ponder this from a feeling based standpoint, rather than the actual deeds, themselves.

"I know her from going out. She's very weird and a lot of people are wary of her, me included after she once followed me around a club all night! So I told him what I knew about her and he became v defensive when I joked about him meeting girls on the internet."

At this point, you seem to be in a mood where because you know this woman as being weird, there really isn't any alarm and so you took it upon yourself to give him insight as to who she is .. your words don't suggest that you are jealous in any way, or worried because you said, "when I joked about".
However, because he became defensive, then suddenly, your mood shifted to ..

"So...I did a bad thing and looked at the messages"

It's apparant that what drove you to look at the messages wasn't the fact that he was actually talking to this woman, rather because it offended you that when you joked about this, he became defensive. So, in reality .. it wasn't his actual deed of talking to this other woman that got under your skin, rather, he made you feel as though you didn't know what you were talking about when you said she was wierd .. you felt like he wasn't trusting your judgement.

"but he does end them with kisses, which wouldn't bother me if he actully knew her."

By your admission .. this wouldn't bother you, yet, you were, indeed, bothered. So, again, it's not his actions that upset you.

"He didn't lie, and admitted he had."
"I didn't say I'd looked on his msgs,lied and said a mutual friend of me and her mentioned something when I asked him"

He told the truth and you lied.

copperhead, do you see where I'm going with this? His deeds are NOT what's causing you anguish right now .. it's yourself and how you are feeling about yourself, how you handled yourself.

"What hurts me the most is that he knows things like this are my one area of weakness and he's just gone and done it."

He's just gone and done what, exactly? If it wouldn't bother you if he ended a note with a kiss (as you admitted is indeed the case), and if he didn't lie to you, rather told you the truth .. then what exactly is this area in which he should know would cause you this grief?

Let me ask you .. would it bother him if he knew you lied to him?

That IS what this is about, isn't it?

Not that he did anything .. because he really didn't and was honest .. the problem here is that you betrayed his trust by first snooping and then lying, which, likely wouldn't upset you if he had lied .. but, he didn't .. which leaves you to be the one in the wrong. And this is what is bothering you.

Most people believe that they relate to others in a pattern .. actions cause reactions.

When in fact, most of the time .. we are actually reacting off of our own actions.