@Zoe191,
Zoe even 18 months is a short time, as Eh-Beth suggested, both living at home and then jumping into marriage and a house.
Even you've confirmed you don't "really" know him that well, others could see traits that you didn't see.
If I was to guess, I would guess that everything moved too fast for him and he does not feel that he can afford a Mortgage or the stress that goes with it and felt pushed into it, so declined the relationship in the end.
Getting engaged is a sign in my opinion of committing to someone, letting them know that they are the one that they want to spend their life with the progression from there is a joint thing and based on a lot of things, finance for one.
I realise at 26 you would be thinking about "family" but we as women can push our significant other halves into fear of going to fast.
I don't know what he does for a living but the mere fact that both of you live at home due to affordability suggests that a Mortgage would be to stressful at this point, saving up for a house significantly over a couple of years would have been perhaps the better way to go.