So you want to write what John Falstaff calls "the book of your heart," and you can't really conceive of writing anything else until it's done? And you don't have any kind of ending in mind for it, either?
Those are two separate problems.
First, to write a story, you should have an ending in mind. Your ending can change, it can increase in detail and complexity, it can need to be revamped quickly - but if you don't have an ending, you don't have a story, just a series of events that happen. So spend some time and think of a few. I have this issue with a potential story of mine - no ending I can think of is right. That story is on hold until I figure it out.
Second, everyone advises short projects so that you can gain the skills you need to do all the parts of writing and publishing a story that you've never thought of without having to make all your mistakes on the baby of a story you've worked on for 10 years. But if you really can't write anything else, then by all means go ahead. But take John Falstaff's advice and put it in a drawer after you're done, and go back to it after you've done a few other things. I guarantee that there will be major overhauls that you will want to make to it after you've developed your skills further, and you will be glad to not have released it right away.