internet porn / masturbation

Ok - your bf is possibly obsessed with a media genre - professionals paid to produce interesting material, at least interesting to him right now. It doesn't reflect on your worth, or even on his, but if you want to get past this you're both going to need to figure out what he needs and why he's pursuing this method of filling that need. You also need to work out what you need from him on an ongoing basis - what's negotiable for you, and what isn't.
The reason I stress that this is a media genre is that porn is a pretty emotionally loaded subject for a lot of people - they feel like sexual subjects should be exclusively conjugal, or that porn is morally objectionable. But if someone were hooked on scaring themselves with horror movies (and indeed much porn is as much about fear and power as it is sexuality), I'd still see someone watching produced media for the purpose of achieving an emotional response. And most men masturbate a lot more than most women are aware. When that gets to the point where it's cutting into your sex life together (I think this is what you mean by his saying he's tired) it's too much, but it's a hard cycle to get out of.
You're going to have to bring the subject up because trying to keep your knowledge a secret is going to eat you up, and it won't improve your relationship anyway. There's a good chance your bf is having trouble approaching real life sex, and this has become his substitute as well as a general-purpose way to space out and relieve anxiety. And if that's true there are probably some deep reasons for that. But until you talk to him, you won't know what the deal is.
By the way, you mention "cybersex", and I'm not sure if that's what you mean - I'd define that as interacting with another person online for sexual pleasure. The difference with porn is that it's (very) solitary media consumption. Be clear with yourself - is your bf cheating on you, or is he going it alone? It may not seem like it, but they represent very different things to the person pursuing the behaviour.

I couldn't have put it any better. That was an excellent break down. We need to be careful. Porn is not cheating, but it is dangerous, because the escape is almost the same as many substances that draw someone to addiction. I am sure your bf still loves you, he just has been pulled in the same way a coke addict is. If you approach it this way you'll succeed and you will both grow from it.