When the end that you didn't trim far enough back starts to pull away and gathers too much dirt to stick, pull off the whole thing and reapply with both strips in tact. That way if the sill isn't perfectly flat the strip on the outer edge can ensure that it's sealed better. Splitting option is for very narrow spaces like storm windows etc. This stuff may not be what you need though. They make self adhesive weather strips out of a memory foam material that can go from 2-3 inches to less than half an inch to accommodate multiple space coverage needs in one area. I'd give that a go if you're a renter and don't plan to take the advice if others and have your window reinstalled properly.
THANKS, I NEVER THOUGHT OF PUTTING RUBBER STUFF LIKE THAT UNDER THE WINDOW. I WAS THINKING I WOULD HAVE TO PUT SOMETHING ON THE WINDOW SILL PART. IM MAINLY INTERESTED IN KEEPING SMOKE OUT. WHEN SMOKE ISNT HERE I FEEL A BREEZE COMING IN HERE. SOMETIMES IT MIGHT BE A SPIRIT BUT OTHER TIMES I THINK ITS WIND FROM OUTSIDE AND IT SHOULD NOT BE GETTING IN HERE THROUGH CLOSED WINDOWS.
That dirt build up looks like black mold. The frame is bulging because of high moisture content, either from the outside, or inside. You should have someone do a mold test on your house, if there is black mold visible, there is likely more hiding.
Your frame is bowed because your window has not been fitted correctly in the first place, nothing to do with moisture if it is a upvc window. and should have had seals already fitted to it that don't seem to be there?