Am i heading in the right direction?

[donnaf]

Hi,
I'm 40 years old, and have been married to my H for nearly 9 years, we've been together 11 years, we have 2 children (7 & 4), the last couple of years our marriage lost its pizazz, we knew it was happening but didnt really know how to cope with it.
Last september he started to text a younger woman he met through work, by the time i found out in jan there was over 800 texts that he had sent. Anyway the day after my sons 7th birthday he comes home from work and announces hes leaving. He goes straight into her arms (he says there was no physical side till he left me!), within the first week she gets pregnant! Beginning of april he decideds he wants to come home, confesses everything and leaves her to go through a termination on her own (by the way she left her husband for mine), anyway he last a week at home, but i was a complete doormat!
I decided to take a break at that time, booked tickets to go and see my cousin and left the next day. Returned 2 weeks later and there was a lot of bad feeling between us. So i cut the contact apart from saying hello when he picked up/dropped off the kids.
All of a sudden, about 10days ago we just started to talk, and got on really well, and he started sending me little text messages with a little x on the end, i responded to some and not to others, this annoyed him!
Wednesday he came round and spent afew hours with us and then asked if he could come back later. Then sent a little text to say it was nice to see me smile. Anyway yesterday he took the day off work to come and see us but i was at the top of the garden and couldn't hear the phone or door bell and he got really stroppy. It ended up he text me at 4.30 in the morning to say he was upset with me. I've spoke to him today and apologised but did explain where i was.
Now he has admitted that he wants to play happy families with me but wants to go home to her.But he has also said he hasnt told her hes getting on better with me.
So what do i do now - carry on cultivating the friendship or set an ultimatum.
Any advice would be appreciated, DB isnt a well talked about subject here in the UK!