Since Comets have tails after millions of years why wouldn't the comet it self dissipate into one large tail?
It will eventually dissipate. Comets lose some of their mass every time they get near the sun. The more volatile components evaporate more quickly than the rocks, so over time the rate of outgassing goes down and eventually all you have is a bunch of dust and rocks flying in formation. The reason there are still comets around is that comets only rarely come near the sun. Mostly they orbit on the most remote fringes of the solar system, so cold that any molecule they happen to run into sticks to them. They don't have tails out there, they are just big dirty ice balls. Comets can only come near enough the sun to create a tail if their orbits get disturbed in a profound way, something that doesn't happen often. Once a comet starts orbiting near the sun it won't last long, at least on a cosmic time scale.