You can't be 100% sure unfortunately. I do hope you don't think i was being OTT or mean in my comments, its so hard to put it the right way in text without coming over all huffy and stuff.. didn't want to offend.
IMO you are heading for an amicable seperation which is great and will be a lot less stress on you and DC's..if you have any.. how do they feel about it? Have you discussed it with them yet?
You will miss him..some days you will regret leaving him tbh, the grass might not be greener then you will feel terrible, i am only being honest.. if the new DP doesn't measure up to your DH, if you compare them, ect.
I cannot advise on this, as have had only 2 relationship breakups where there were kids and violence involved so it was easy for me not to miss ExP's.. one man i went out with cheated on me and broke my heart but we are still friends, and he's lovely, i have sort of knocked our friendship on the head a bit because DP didn't like it, even though i would never go back to my ex ever.
I hope you can find the best way forward for the two of you whether you decide to split or stay together.. and sometimes, the shock and fear of losing you may shake up the behaviour of a partner so your DH may be making an effort..but not allowing himself to be emotional towards you in case you DO decide to go. He may hope silently that you'll change your mind, i don't know. It must be sad for him as well to have the certainty there is someone ready in the wings to step straight into his shoes
You have had time to prepare emotionally, don't be surprised if he hasn't..be kind as you can and gentle with him no matter what you decide. But well done in a way as you have been open with him to the point where it won't be a devestating shock if you leave.