What am I doing...?

I've been in a casual relationship with the one guy for over a year and a half. Golden rules were: if you develop feelings... it ends (the sex part). If you meet someone else... it ends. If you have sex with someone else... it ends.

This has suited me fine till a year in... where, naturally, I developed feelings. I told him that I loved him, to make him aware... it didn't end. We continued. It got to the stage where I was staying with him every night, per his request. (I was more than happy to oblige, obviously).

He went on holidays to Thailand for 2 weeks (messaged me every waking moment). When he got back, obviously, I was happy to have him back and tried to fall back into the old routine... all of a sudden he wants space. I found this out after a massive argument.

So I have him space. I love him, so the age old saying "if you love it, let it go". Within 24hrs, he's messaging me. I've reminded him of space. And he still messages. I said now I need space to get over this.

He reminded me one day that he was supposed to give me space, but continues to talk.

I rejected his offer of cuddles one night (because he knows I struggle to sleep without him) and he got a little upset, reminding him he ended things. Then the next night, he proceeds to offer up sex, even though, we've decided it's done and over.

He can't seem to leave me alone, even though he knows we need space to deal. I love him to pieces, but I'm not sure what to do. I can't cut him off completely because he's also one of my best mates, and I his.

HELP!!