an ex constantly pressured me for BJs; now I always avoid them but feel guilty about it. What do?

I dated this guy about four years ago, on and off for maybe nine months. The sex was pretty decent, but he absolutely loved blowjobs and wanted them constantly. He also always wanted to come in my mouth, which I wasn't comfortable with, but he frequently tried to talk me into it anyways. I did my best to keep him satisfied, but eventually I really didn't want to blow him because he constantly asked about it, and would make me feel guilty when I said no. I always felt like I wasn't good enough at it and like he always wanted more than I could give. Before I met him, I was loved giving blow jobs, I loved watching the guy's face and seeing how good I was making him feel.

Since we broke up, I haven't gone down on anyone. I'm finally dating a really good guy (and having the best sex either of us have ever had), and he's asked me to blow him a couple times. The first time he asked, I explained what happened and he was very understanding about it, tried to show me how it'd be different and how I don't have anything to worry about (he was fairly inexperienced when we met, so he says I basically have no competition, and also said that he doesn't want to come in my mouth anyways). We've been sleeping together for almost two months, but I still haven't gone down on him. What he's said and how he treats me day-to-day has made me feel a lot better about this and it's made part of me really want to go down on him.

But I keep freezing up. Saturday night, we were drunk at his house after a party, and I dragged him into his bedroom and after a while, I was totally okay with doing it. But then he commented that I looked nervous, and I broke down and we had a two-hour conversation about our past relationships and insecurities, which really helped out I feel about this. When I left that night, he told me that I better suck his dick Sunday night (we have a slight dom/sub thing, so this was totally okay with me). The following morning at work I kept thinking about it and was actually looking forward to it, but once we were in bed that night, I couldn't make myself do it. When he asked me about it, I said maybe in the morning, but didn't follow through with that either.

What should I do? He's not pressuring me at all about this, and is being very understanding about everything. I know it's not my fault that I was treated badly before and have hang-ups because of it, but I don't understand why I keep freezing up and I'm getting frustrated with myself. Advice please? Has anything similar happened to anyone else?