Sometimes the problem is that you're starting too many new pieces. To write a full-length novel, it's important to buckle down and concentrate. If you get 5,000 words in and then start something new, you're never going to finish anything. You have to discipline yourself to keep going. Back when I was writing short stories, I had dozens of stories in my "in-progress" folder. None of them ever got finished. I had to learn to keep focused. If I got a new idea, instead of starting a new story, I would just add the concept to my "ideas" folder. Once I started forcing myself to finish one story before beginning another, I started making progress. Now, I have more than 50 (finished) short stories to my name.
It's possible there may be flaws in your novels that your subconscious is trying to warn you about. This happened to me last year--I wasn't able to write more than 10,000 words and I soon realized it was because I didn't know know the main characters well enough. They were like cardboard, until I sat down and fleshed them out. So look at your novel with a critical eye. Perhaps there's a major plot hole that needs attention, a subplot that needs to be added, or characters that need to be developed further.
Or perhaps you need to write an outline, to guide you and tell you what to write next. Many people write by the seat of their pants, but a lot of people need an outline to follow. Using an outline doesn't mean you're limited to what's in the outline. Think of it more as a rough sketch beneath an oil painting--you can still paint whatever colors you want. You can do whatever you want with your story, but an outline will help keep you on track, so you don't wander aimlessly. Truthfully, my novels usually end up vastly different from their outlines. I'll discard entire subplots, or add new ones. I let the story evolve naturally, even if that means departing from the outline. But when I don't know what to write next, I go back and look at the outline, to see what still needs to be written.
One last possibility is that you're simply not ready to write novels at this point in time. There's no shame in that. Some people write nothing but short stories and novellas. Others prefer to write thick novels. Some (like me) started out with short stories, then moved on to novellas, and finally graduated to writing full-length novels. There is a big difference between them--a novel isn't simply a longer short story. The structure is entirely different. It takes practice to be able to write one successfully. In fact, most published authors do NOT start out by selling the first novel they wrote. They relegate their first (and sometimes their second and third) novels to a trunk in the attic and don't sell anything until they've had plenty of practice.
In other words, don't give up! Figure out what's stalling you, and work on fixing the problem.