I'm someone who comes up with a lot of ideas. When I was a teenager, I used to constantly get excited about new projects, and I found myself often getting sidetracked, ditching existing projects before they were finished so I could start something else. It took me years to learn the discipline to choose and stick with a limited number of projects. They say that ideas are a dime a dozen, but I would say that some ideas are definitely worth much more than others. Your strongest asset, as a creative person, is to develop the ability to recognize which of your ideas have real potential, and are truly worth spending your time on.
Nowadays, when I have an idea, I often write it down and set it asides. If it's a really interesting idea, I'll come back to it later, and maybe flesh it out a bit. I think it's important to figure out the details, but also to criticize your own ideas a bit, by thinking of them in adversarial terms (how could this fail?). This is a gradual, iterative process. The more fleshed out an idea, the more layers of adversarial testing it passes, the more it becomes worth spending time on. Ultimately, before you invest any real effort in a new idea, it's also worth thinking about whether you have time to do so, and how this would affect the other projects you're working on, and the people you're working with.
Once I've sufficiently fleshed out and tested an idea in my head, if I'm still excited about it, I'll want to discuss it with other people. That will help me get useful advice, outside perspectives on how to improve the idea, and maybe even recruit some help. At this point though, the same thing always happens, I'm inevitably going run into one or more people who give me a variant of "it's been done before". These people will point to some existing project that they believe is similar to what I've just described. Sometimes they mean well, and are just trying to help me differentiate my project or help me avoid spending effort on what would be a dead end. Sometimes it seems like they are cynics who can't stand to see that I'm excited about something. I try to avoid working with the latter kind of person.
The most cynical among us would tell you that in movies, literature, and music, there are no more new ideas. It's all been done before, "what's old is new again", all possible thoughts have already been conceived, and we're doomed to forever rehash the same old ideas over and over again. There's some truth to it: how many songs and movies are about boy meets girl, or the feelings that follow a bad breakup? The more songs and movies are written, the more various concepts and ideas have been explored, and the harder it becomes to come up with something truly groundbreaking and innovative. There is one caveat to this, however, which is that the world is changing constantly. What will love be like in the year 2073? It might not be quite the same as in 1982.
Your idea isn't novel. Any software-related idea that you've had, someone implemented it on a Lisp Machine at MIT back back in 1977. Unfortunately, the backup tapes have been lost in a fire and there's no evidence left, I have no material proof, so you'll just have to take my word for it, someone did beat you to the punch.
It's happened many times that someone told me that "it's been done before", without being able to actually provide any reference to a similar idea. It's happened that, after I did some digging, I found that whatever idea the person cited was only superficially similar to what I had suggested if you squinted really, really hard. There's also been times where someone pointed me to an existing project that was a very poor execution of my idea and basically told me that because this project had failed to take off, the idea would obviously not work.
Before you embark on a project and really invest yourself in a new idea, you should do some research and look at what's already out there. It's quite possible that you'll find that your idea is not as novel as you thought it was. Still, I think that in the world of software, the worldwide context is always changing. It's quite possible that as you start doing some research, you'll find that others have tried to do something similar to what you want, but they didn't execute well, or they simply had the right idea at the wrong time. Just think about electric cars. There have been many failed attempts (dating as far back as the 1890s) before there were successful commercial products. Finding such failures will provide you with an opportunity to learn from the mistakes of others.
Ultimately, having a truly novel idea might not even matter. If you have an idea for some kind of accounting software, and you find that your would-be competitors are raking big profits selling something completely broken, you might be able to eat their lunch just by executing reasonably well. It can also be a fun learning experience to recreate an existing system without necessarily looking to innovate on its design. If you have an idea that really has you excited, go for it. It's important to have realistic expectations: you may not succeed, but you will definitely learn something along the way. One thing is for sure, which is that you'll never succeed if you don't even try. What's the point of living if you're not having any fun? Explore.