So this is a question that millions of people struggle with in every career path, and unfortunately the correct answer is the one nobody wants to hear: "just do it" but what does that mean? Obviously you can't "just become a professional manga artist" there are many barriers that you have break through first, and the issue is most people stop there.
A big example of this is youtube, many people say "I want to start a youtube channel and have a career as a content creator, how do i start?". Well when you think about it, how did PewDiePie start? He just did it. He made a video, earned $0 off it, and got no viewers. But he made another video anyway, and he kept doing that until a video suddenly blows up, 1,000s of views, and suddenly sponsorships appear and eventually youtube creates the partner program.
We tend to set goals for ourselves specifically so that we can fail them, almost like how certain people (not all) who never learned how to deal with rejection will be attracted to married men and women because rejection doesn't sting the same way from someone who is obligated to reject you. Like many first-time game designers say "I want to make an MMORPG" in the scale of world of warcraft, but when they realize they only have the capacity to create a mobile game in the scale of Angry Birds then they just give up, rather than creating anything.
Focus on what you can do and eventually you can do more. Like the old saying, a builder doesn't think to himself, "I'm going to build a house today", he thinks "I'm going to lay down a brick, build a door, finish the pluming, or wire the electricity" he thinks in terms of what he can do. Eventually he will build a house, but right now he can build a door.
You can't just become a Professional Manga Artist, but you can draw a picture of a character, a picture of a fight scene, two characters kissing, an environment. You can make a 1-page comic. You can start there and trust me nobody will care about the very first picture you make, you won't get paid for it, and your future self won't even like it because (ideally) you'll have grown so much as person. You are building your goal as a professional manga artist each time you do something.
Everything you've talked about are real barriers that you will encounter, but you cannot allow these barriers to be an excuse to do nothing.
You have too many ideas - Draw all of them - write down all of them - do them one by one
Get tired of an idea - publish it unfinished or leave it there until you're interested in it again, start the next one.
And the thing about motivation is that we are all motivated to do what we really want to do. A great example is the common meme format where a girl asks her boyfriend to hang out and he doesn't want to, but when she mentions that she's home alone, he suddenly has the motivation to cross mountains to get there. When you really want something you're going to make it happen, but hear me out.
Sometimes you have to do it in order to be motivated to do it. And that probably sounds crazy, but there might be something in the process of becoming a professional manga artist that you really want that you didn't know about until you tried it.
My personal experience with art is just like this, I thought that I had a passion for Art but I was rapidly losing motivation for it, but deep down, I had a greater passion for "Design", and I really enjoy the role of a designer. In order to make art usually you have to "design" it: meaning you have to figure out what it is, what looks nice, and what makes it interesting whereas the artist part just deals with the physical work of drawing it. This is something you're going to run into as a manga artist because when you're figuring out how to setup the panels and the size of the book (or dimension on a web page) you will be doing Graphic Design, not illustration. In the process of building this career you're going to end up doing things like social media marketing, customer service, computer science, probably a bit of chemistry and logistics if you were doing hard copies, and somewhere along the lines you're going to find something you really enjoy, and you probably just haven't had the chance to try it yet.
So if you're not motivated to actually doing the career, be motivated to finding your motivation.