How do I gain his trust back?

I met a guy right after having a difficult break up he helped me through it i fell for him very very early on he felt the same but we took our time to name it but in the process i was talking ti another guy we flirted and there were talks which friends shouldn't be having the day i decided i needed things serious with the guy i was crushing on i stopped talking to him i asked him out and we have been daring since then but now he found out about it and he ended the relationship how do i gain his trust back?

How do I gain his trust back?
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