?..
Wait.. She knew he was going to kill himself on this day, because he told her someone prevented him from killing himself on that day, but she couldnt have gone back to prevent him from doing so, without him telling her first..
Wouldn't that be a paradox then, since he should've died, since she couldn't possibly prevent him from doing so ?
Not quite,
one of the prevailing theories on time-travel is that
if it was at all possible, it would also be entirely impossible to change the past/future. So if someone went back to kill their grandpa, they would inevitably fail because their grandpa wasn't dead originally. And originally they had gone back in time trying to kill their grandpa.
We can move it down to fewer dimensions (yes, this theory is in large part from the idea that time is an actual fourth spatial dimension), You have a 3d-shape. The shape itself (in 2d) changes over z (the third dimension), despite the shape (in 3d) being rigid as z passed. If it at any point actually loops back around you will in 2d see it as 2 isolated islands (and if it touches or connects with itself those islands will touch at that z). But this was
always the case at z=0, the "time-travel" (or rather, "3d-travel") did not change anything and create two "3dlines" ("timelines") or anything like that.
Now replace everything with being 3d slices over a fourth dimension "t" instead. At t=0 the time traveler always existed and their actions were part of the four-dimensional shape looping back on itself.
Instead, the only paradox would be if she had gone back in time to
encourage him to suicide, making them never meet. That would be equivalent to the portion looping back through the extra dimension intersecting itself in such a way that a discontinuity exists. And they are in fact instead 2 different shapes. In that case they never were the same shape, and it can't be considered as having looped back on itself in the first place (or however you want to explain it).
But while time is tied to spatial dimensions in a spacetime continuum, it's not absolutely certain it's exactly the same as just an additional dimension (which was how it was described above). And some theories treat it instead more as a passage of time, than a movement in space. It's in those theories you get stuff like the
sawtooth snap,
N-jump (which arguably is just a finite sawtooth snap, though arguably sawtooth snaps must be infinite themselves even if they don't self-recurse and n-jumps don't exist at all), or
infinite loops (paradoxes).
Or ("and"? The above 3 concepts aren't mutually exclusive to this) parallel timelines being spawned from thin air branching from an existing one that still continues on - just with the time-traveled matter removed poof out of existence, when they traveled).