I left then I came back?

So I left my boyfriend of 3 years to date other people for 9 months. It wasn't all it was cracked up to be and I constantly missed him.. so after 3 months of singledom and still being friends with him and having lunch and stuff regularly, I asked him to date me again. Only... no matter how hard he tries, be can't seem to get comfortable being with me. I think he is afraid of being left again and it caused him a lot of pain to be left but he stayed my friend because he hoped I would come back one day and I did.. but he is very distant now. He's still very sweet, but there's no intimacy or anything anymore and I'm trying to fill the void between us and get back to what we used to have...

How can I make him feel safe with me again? And how can I prove to him I won't leave again?
I left then I came back?
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