I must confess that I used to have trouble finishing what I wrote, and in fact, it wasn't until my first book, Tiger Eye, that I was actually able to complete anything at all except for poetry and non-fiction creative essays (as well as some fan-fiction). I don't know why that was the case, though it caused me a great deal of frustration. I had so many ideas! So much that I wanted to write! But I would get a couple chapters in, and then boom. Full stop.
What was the difference with Tiger Eye? It's difficult to say. Could be sheer desperation combined with inspiration. Could be I was finally old enough, had read and written enough, that I had the confidence to allow my subconscious that extra push. Either way, I sat down and wrote. And I felt consumed by it. I couldn't stop. I didn't worry about who was going to read the story, either. I just wrote what I would love to read.
But now? Sure, I hit dead ends. But not because I'm bored. Though if I find myself bored, it means I've done something very wrong in the execution of what has come before - either with story, character, etc. -- and I must go back and rewrite, start all over again. Which I've been known to do anyway, but only because I've realized that something needs to be fixed -- sometimes drastically.
As for staying interested, though...that's also not much of a problem. Again, if I lose interest, it means there's something wrong. I try not to let it get to that point, and while I don't have a particular method I use, I will say that (in my somewhat limited experience), the best way to keep a story interesting -- and the writer and reader interested, as well -- is to have really great characters. Invest your characters with life, let them lead you in the story, and I think you'll find your job as a writer becoming much easier. When I wrote Tiger Eye, for example, I didn't know what was going to happen to Hari and Dela -- but I couldn't wait to find out. I didn't know how Elena and Artur were going to save themselves in Shadow Touch -- but I was so invested in them, I had to sit down and write until I knew the answer.
Just relax, though. Don't be too hard on yourself. Stay joyful in the writing. Have fun with it, let loose. Fall in love with the characters. Put yourself in their shoes and imagine the adventure of a lifetime. The rest will take care of itself, eventually.