I'm sorry to hear you're going through a bubble of insecurity. I live long-distance from my GF and I know entirely what that is like.
So, how old is your boyfriend? What stage of life is he at? Why is he thinking of going to Australia? Sometimes these things can be a stage of life thing, a personality thing, a lack of commitment thing... there could be a multitude of reasons. The most important thing is that you have noticed is the disparity between your motivation and commitment regarding the long term goals of your relationship and his.
When I met my girlfriend, I had no intention of moving abroad. I told her it was very unlikely to happen, because I was very connected to my home country. After a year, I changed my mind. Two years into our relationship and I am desperately trying to move over there to be with her. But, I'm 28, I've done the partying, travelling, letting go of parents, etc.
Now, let's say your boyfriend is completely committed and in love with you, but just not much of a planner. That's a current personality thing. My GF is 38 and hates planning. She's very free, artistic and not a natural organiser. I adore these qualities about her. Her favourite phrase regarding visas is "it will all be ok". I love it when she says this, because she has a way of making me believe it. But, it means that I'm the one doing the research, the planning, the visa applications, the stress of getting questioned at the POE, the long flights, etc. So, we actually want the same future - we just have different ways of approaching it.
How does your boyfriend feel about your partner? Does he really want to live with the two of you? I've got to be honest - if I'd been with my GF for four years and she met a new partner and moved in with him/her, I'm not sure I'd want to live with the two of them.
How often does your boyfriend come to see you? Is there any pressure or a ticking time bomb? Has he ever been sent to secondary questioning when trying to enter the country? If not, is there a hurry?
If you're both in your early twenties, it's the time for exploration and adventure. Your boyfriend might not be ready to leave the UK. He may never be ready. Have you looked at options to move the UK? If not, why not? Why does he have to move to the US?
It's interesting that you think moving in with your partner will affect your LDR with your boyfriend. Does it have to be that way? Will you have any time alone, or a separate room to make Skype calls from? Do you absolutely have to live with your new partner?
As for your partner....
So you are a secret girlfriend to him? His friends don't know about you? Or he's just more demonstrative with her on FB? That's a hard one. Seeing that kind of thing can be very hard. Ultimately, it's potentially unfair to ask him to curb it. A better option would be to take him off your facebook friends list - then you don't have to see everything. Or, of course, better yet, would be for him to acknowledge you in the same way.
Why aren't him and her living together? How have you two come to the stage of cohabiting if she's not entirely happy?
Sometimes being polyamorous means that you end up dealing with judgement and negative options from others. It's a shame when that happens; but have faith in your partner. Don't let what other people think effect you. All that really matters is how you and your partner feel about each other.
Overall sweetie, it sounds a little like you are in need of some stability in your life and you are crying out for it. You're lonely in a certain sense, because your partner's public affections are directed towards his other girlfriend.. and your boyfriend is long distance and not sure what he wants to do with his life. Honestly? If it were me? I'd give it six months and review. I'd make the transition from college and get myself into a settled place. I'd think carefully about living with your partner and how this may effect your relationship with your boyfriend, if at all. I'd talk to your boyfriend, make sure you're not pressuring him to move and work out what he really feels. I honestly think you should give it some time.
me: female, 29, homoflexible
GF: my primary girlfriend, 39, heteroflexible
my metamour = Hubby (GF's husband)
3 year, open poly V, long-distance between the UK and USA