When you make your intention about achieving something, your attention to that something not yet being achieved keeps it from coming into being. When you make your intention about feeling how you want to feel, your attention to feeling that way makes you a match for drawing in the cooperative components for how you want to feel.
Think about it this way. The same energy that attracts a piece of metal to a magnet is the energy that keeps it there once they're united. The magnet simply continues to be a magnet and the metal naturally continues to be drawn to it.
So the same energy that attracts a new relationship into your life is the energy that will maintain it. And the reverse is true as well. The same energy that would maintain the relationship of your dreams is the same energy that will attract it.
If you were in the relationship of your dreams, and confident that it's here to stay, then every time you thought about your relationship or your lover you'd be happy. All kinds of yummy thoughts would naturally bubble to the surface of your mind. It would be effortless and absolutely joyous.
Now imagine you're in the exact same relationship, but you're constantly worried about losing him. Half the time you're noticing all the yummy things you can appreciate about this dream relationship, but the other half is spent analyzing everything that you're doing and trying to make sure you don't mess things up.
Which approach is more likely to maintain the relationship of your dreams?
In both scenarios you're in the exact same situation, but your focus is creating two vastly different experiences.
In the first scenario you're focused purely on what you desire and appreciate. It's like you're telling yourself "Oooo, I desire this. And I already have it!"
In the second scenario you're keeping score—taking note of all the you love and appreciate, while guarding against and fretting over anything that might bring this relationship to a premature end. It's like telling yourself, "Mmmm, I love this. Uh oh, what happens if I lose him?" And with that sort of focus, you'll eventually be focused on "Uh oh, what happens if I lose him?" long enough to attract actual evidence that suggests you might lose him. And then your focus on that will strengthen that possibility in your vibration until eventually you'll lose him. Then you'll be apart and miss each other and get back together. Then (unless your focus changes) it will repeat itself and you'll separate again, only this time for longer because the prospect of getting back together is being tainted with the potential pain of breaking up again. In short, you'll be offering an "on again, off again" vibration and attracting an "on again, off again" relationship.
And is this not what you've been experiencing in your quest to manifest this relationship? You do some visualizing and feel great, then you notice that he's getting closer and start worrying about all the things that could go wrong, so he grows distant again. You find yourself feeling inordinately good on holiday and manifest some positive evidence, but then you start worrying and so the synchonicities cease. Then you find a way to feel good again and he grows closer—thought not as close as before. You start noticing small things that seem to be working in your favor then he resigns and you start to doubt whether it's ever going to work out with him.
The pattern that I'm seeing develop is that when you focus purely on feeling good, he draws closer to you. But then you notice him drawing closer and start worrying about whether or not he's right for you and your vibration pushes him away. Then you start feeling good again while on holiday and release resistance to your worry that he might not be right to you. Synchonicities arise from that that help you feel better about him being right for you, but then you start worrying about attracting him and it pushes him away. Then you find a way to feel good again and he draws closer, but as he draws closer you start comparing it to all the other times he started to draw closer and how that didn't work and so this time you focus even more on "his barriers" and the other obstacles in your way.
Summation: The more you focus on pure joy, the closer you get to what you want. The more you focus on how close you are to what you want, the more the absence of what you want dominates your vibration and thus pushes it away.
Solution: Focus purely on feeling good.
When you can be happy with or without the relationship you're desiring, then you'll know you're a vibrational match for it. But do it for the joy of feeling good. That way your attention will be on focusing in a way that allows you to feel good (something you can control) and you will find it much easier to continue feeling good. And from a place of feeling good you can't help but manifest a life that reflects all of the yumminess you so easily feel.
PS: Sometimes I wander when first tuning into someone else's situation and trying to articulate what I feel would help the most. So feel free to ask for clarification and fill me in on any added details you think would be helpful to share.