In my experience both works. When you are absolutely sure that something will happen in the future (for example you flying to another city), it will happen, because it's your expectation. But that only works well for things that are easy to expect. With the specific person it's a different story. I would focus on him being your boyfriend now, because it's hard to expect someone coming back to you with 100% certainty. And you have already trained yourself in the relationship vibration, so it's less likely for you to fall back in old thinking patterns, when you are back together.

Letting go doesn't mean not thinking about it. It means letting go of all the struggle, resistance, negative thoughts, thinking about how it is. When you are focused purely on the relationship you want with him and live it in your thoughts, you can think about him all day. But it's likely that you can't hold that vibration all day every day. Just let go of the need to think about how it was or how it is now.

Both are great advice. Definitely do not try to think about what they may be thinking. That's a sign of attachment to the outcome. It's thinking about the how's and why's, leave that part to the universe and just trust that it's all working.