Welcome to the forum. I'm so sorry to hear that you're in this position with your relationship. While I don't have much experience with this scenario, we are the same age (25), so maybe I can offer my perspective and advice.
Sometimes I feel like there are different levels of adulthood, and occasionally I come across something that feels like it's from a more advanced stage than I'm at, for lack of a better term, and dealing with the ex-wife and kids of a partner would certainly qualify. This isn't to say that the age gap between you and your partner is wrong or can't work, but it does mean that he has a responsibility to be attuned to this age difference and to the very different circumstances of your lives (in his case, an ex and children).
But the problem is that instead of being extra careful and considerate in a situation that calls for it, your partner has shown you disrespect and proven to be untrustworthy. Without knowing too much detail of your situation, it seems to me that this person has taken advantage of your patience, love, and affection, to the point where it has impacted your mental health. In my opinion, relationships worth staying in do not involve badmouthing to an ex-wife or feeling the need to check phones behind backs, period.
All this to say that it isn't exactly that you are too young, nor that he is too old, but that his behaviour is totally inappropriate under any circumstance, and is made even more so because of this age gap. He has had an entire adult experience of a marriage and raising children that you simply weren't privy to, and instead of attempting to bridge those two parts of his life, it seems he has decided to keep them compartmentalized and antagonistic.
I'm also an expat who moved to Australia recently, and I know that it can be lonely and difficult. But the key is to fill your life here with people worthy of your love, even if it takes time. Apologies for the rambling, but those are my two cents. You are always welcome here to talk, and would love to hear more.