ok, so it sounds as if you are generally the kind of person who falls hard and fast. So two months previous you had feelings for someone else and actively pursued this person but were rejected, and you then wrote in your diary about your hurt feelings, and presumably about your feelings for this bloke.
Two months later you then got together with your now dp, the how is kind of irrelevant, but you were friends so already on each other's radar at that point even if only platonically. The relationship has run very quickly and just a year on you are already engaged, so another sign that you fall quite fast and your relationships develop quickly.
Now this bloke has discovered that just prior to you falling in love with him, you were in love with someone else, but they rejected you, so now you're with him, and in his mind, you've rushed into a relationship with him because the other bloke wasn't there. And in his mind he may also be questioning what would have happened if the other bloke had come back at the beginning of your relationship. Would you have gone to him? or even been conflicted?
You haven't done anything wrong but his feelings of hurt and upset aren't unjustified either.
So now what you need to do is to have an honest discussion with him. You don't have to defend your feelings for the other bloke, but explain to him what you've said here, that when you got together your thought process changed. It's all about transparency at this point. no-one wants to feel that they were second best, even more so a year down the line when they've been building a relationship and then find out something which in their mind means that the only reason their relationship exists is because the bloke you really wanted wasn't interested. Even if that's not the case you still need to talk about it.