0.8% agarose is not sufficiently dense to provide higher resolution, and therefore separation of higher weight bands. Use agarose gel with 2% or higher

From the distance that your dna ladder has run I think that you are running the gell at too high a current.Run at a lower voltage and the larger bands should improve. I think that the most likely reason for all lanes being populated is that you poured the gel too hot and the tray warped so that the comb teethe are touching (or very close) to the bottom of the gel tray. Thus when you remove the comb the bottom of the gel is torn and dna can flow under the gel into the next well. Allow the gel to cool more before pouring and make sure that it is well set before removing the comb. Also watch the samples during loading to see if they are spreading. The blue loading dye will show if this is happening. I agree with Péter Gyarmati that a denser gel would help .0.8% is good for larger molecules but your amplimer is quite small. One thing which has worked in the past is to heat the ladder to 94c briefly and let it cool which has worked for me.