Why are some days harder than others?

I left my ex of 15 years, we had a roller coaster relationship, he cheated, lied, told me he didn't love me but I fought hard to "make it work" till finally enough was enough...we have children together and now I struggle to get him to spend regular time with them and he doesn't give me any kind of monetary support. I started seeing someone else and he's trying to make it out to be that I left him FOR this other man and says I F'd him over because I got with this guy who was a mutual friend...I'm being treated like a queen now and happy in this relationship however sometimes I wake up feeling terrible, depressed and somehow missing something, could it be him or just the comfort of the relationship I was used to?? I find myself overly apologetic to this new man and he keeps telling me, "stop apologizing, I'm not mad/upset with you" He says he can see all the fears my ex instilled in me...am I depressed, can anyone offer some friendly advice?