First, I want to tell you that you aren't alone. I can really relate to your post in all its part. I have "chronic bad luck" with relationships since I know about LoA - seriously. Everytime I tried to make something happen, it just went wrong even though physically everything looked good..for example, I had two guys disappear without any arguments or bad feelings just completely random when I was sure that they were interested in me and "tried" to manifest them. Before I knew about LoA, I got every single guy I wanted. So what's the difference here? I didn't try. I didn't try to make it happen, I didn't think about them so much, I was more like "Okay, I have no control about it, so I just go about my business". The whole LoA thing is kind of paradoxial. When you find out about it, you are like "woaaaaah, I have the power. I am in control. I will make all good things happen in my life" and with exactly this approach you block it all out. I think most people fall in that trap. LoA is about being gentle, letting it flow, allowing it in, trusting, expecting, knowing that everything is provided for you. It's just words, but it tells a lot about where you are coming from. You don't want to overcome resistance. You want to relax into non-resistance.

I really struggled a lot with resistance myself and that about years....a part of it was and still is a general resistance towards life. Not in the sense that I don't want to live or something like that, but more I don't expect life to be good for me. I tried everything to feel better and to get rid of it, but I couldn't, because I wasn't even aware that I have such a belief. So my experience is that every resistance comes from a belief and until you find that belief that isn't in line with what you truly desire, you won't be able to get rid of it (the resistance). Let's take an ex as an example. You can do all kinds of techniques and think positive thoughts and stuff, until you have that feeling of resistance in your body, nothing will happen and it will probably get worse. I use that as my indicator for how close I am. Until I am resistance-free, I know that I don't have to be disappointed, because it's my own fault. You say that you are scared that he won't come back. Think about what your beliefs about him, relationships in general, yourself and the world are that are behind that fear.
Do you doubt that LoA is real? Source of resistance
Do you think that you don't deserve a relationship or him in particular? Source of resistance
Do you think he hates you/doesn't want to be with you or something like that? Source of resistance

I think especially the last one is a big obstacle for many people. We assume that people think and feel a certain way about us. But the reality is that we just imagine what they think and feel about us and accept that as true...that's called a belief. So it isn't true. Whatever you think he thinks and feel is not verifiable for you. You won't ever know. Even if he told you he hates you or whatever, you can never know the truth. If you can get clear on that, you should be able to replace the old belief with a new one. If you affirm every day "he loves me, he loves me" and don't believe it, you will feel resistance. And what's even worse than that feeling is that you activate that belief, so you create more of him not loving you. But if you go about it the way I just pointed out, you are aware of the underlying belief and that takes the power away from it and with it the resistance. Everytime you feel the resistance coming back up, you have once again bought in the old belief or you have still other beliefs going on.

When I started with LoA, I wasn't too much in changing beliefs, because it sounded like so much work. But I think it's the only route to get to where you want to be.

And about the money: That's exactly the way it works. You didn't freak out, you didn't resist, you didn't buy into the lack story, you just knew on some level. I can't tell you why it was so easy that time. But observe yourself on that for a while. It's pretty interesting (and seemingly random) when you can believe stuff without evidence and when you can't or buy in something bad without evidence. It all comes down to your beliefs on some level. It's not always easy to find out what exactly is behind a manifestion, because our belief system is so complex and has so many levels - from very general to very specific. Just learn from that experience and try to recreate it with other things to get a feel for it