When it comes to time travel, particularly into the past, here's how I see it happens.
For the first part of my description, I believe in the existence of parallel universes. For example, somebody is walking down a path and reaches a point where it goes in two directions, and the person has to choose whether to go left or right. What really happens is that the universe splits into two universes, in one universe the person goes left and in the other universe the person goes right.
In this case, imaging the time-line being like a tree, which means the present is like the tip of one branch. All the other branches are like different versions of the present (parallel universes), where certain past events happened differently. If you travel back in time, it's like going downwards on your branch. Whatever changes you make in the past don't change the branch you came down from. Instead, you create a new branch, where the "present" of that branch is different from the one you came down from, due to the events you caused. (heck, even being in the past to begin with would be enough to create a new branch) However, no matter how much you try, you'll never be able to go back up the branch you originally came down from. You could create a new branch that would be extremely similar to the one you came down from, but you can't go back to the one where you first came from.
That's how I think time travel works.