Lost.

I've been talking to a guy for the past almost 6 months. In the beginning he knew that I wasn't looking for a relationship, as time went on he started falling for me, and I knew it. I haven't been long out of my relationship when he came into my life. I explained that I was afraid, and not ready for a relationship. Over time his feelings grew and I tried to hide mine. He always asks me to be his, but when I say that I'm not ready, he becomes very angry. He's also asked me to marry him more times than a few... I tell him I'm not ready & he goes as far as telling me to lose his number, and leave him alone, only to call/text me the next day. He's very attentive, all about me, and he wants to be there for my kids... That's where I shut down because I have to protect my children & I don't want to feel like failed my youngest like I feel like I failed my oldest when my youngest son's father walked out. Whenever he talks about my boys and being there for them I back out. I genuinely love him, his attentiveness, the attention that he gives me, and The love that o know that he has for my children. I honestly don't know that I can be the good woman that he needs. In my mind marriage is only done once and I don't want to fail myself or him when it comes time to marriage. 
I really love him, and I often find myself wondering why me. As I type this I have tears in my eyes. I'm just afraid of hurting my boys.
How can I get him to realize that I want everything that he's offerin, buy at my own pace? Do I let what I have with him pass me?
Has anyone else been in this prediciment?
Am I being selfish?