Thank you for your kind reply.
M. isn't quite the communicative type, he doesn't like to talk about feelings at all and usually falls silent when I try to. Generally, his biggest problem is "having to talk about it", that usually pisses him off more than any situation in itself. I realise that this is generally not ideal for polyamory.
The good thing is, he is generally accepting of anything that comes naturally (rather than being forced to talk about stuff), so I'm going to have to try a more natural way than sitting him down to talk about it.
M. tends to believe that when I'm trying to talk about my feelings that I am by definition "hurt", and therefore he's hurt. We usually experience these talks very, very differently. I usually tend to think "well, that didn't go so badly", while he's usually like "but remember the drama when you told me!" just because I want to share some of my reservations..