Distrust?

Hii guys.

I'm finding it kinda hard to trust my boyfriend of 17 months at the moment...
We've been arguing quite a lot since around August last year, and we very rarely used to, which is bad enough. He'd recently moved out of a room down the road from mine, onto a boat about an hour away from me, and I'm guessing this contributed in some way to the arguing. He made a friend - let's call him L - who doesn't seem to like me, no matter how hard I try to be nice to him, and he tries to cause arguments between me and my boyfriend pointlessly. He still kind of does, even to this day, but thankfully nowhere near as often as before.
Anyway.
My boyfriend left his Facebook logged in at my house one day, and had left it open on his messages folder 'cause he'd been looking for an account number for someone he owed money to - I went to log him out and saw a message from his ex, and 'cause he'd been acting distant with me, my gut instinct told me that it was something important, so I opened it - he'd been talking to her about how stunning she is, and said that she has lovely lips and that he misses her cuddles and stuff like that, and they were reminiscing about the old days, when they used to hang out all the time, and they were loosely arranging to meet up.
I don't know what to do - do I confront him, and have him be angry at me for reading his messages? Or do I ignore it, and pretend everything's okay?
I mean, just because we're going through a rough patch, surely it doesn't make it okay to flirt with your ex?

I've been with him for nearly 18 months now, and I don't want us to split up, but I can't keep dealing with arguing and the fact he's flirting with his ex.
Any advice?
xo