Print gets worse with every layer - Ender 3 First of all, I'm still new to this 3D printing and I've bought myself a second hand Ender 3 with some mods done to it.
I've successfully printed with PLA in different colors but after a good first layer the print seems like it is skipping lines.
I've already tried to:

Change the nozzle
Set the bed level (like a million times)
Download calibration files (e.g calibration squares)

I have used Cura with the Ender 3 Pro profile:

I print in PLA at 200 °C
A print bed temperature of 60°C
Cooling fan at 100%
Layer height of 0.2 mm
Line width is 0.4 mm from a 0.4 mm nozzle
Printing Speed is set to 50 mm/s for walls and 25mm/s for infill
Retraction is 6.5 mm/off at 25mm/s

I really don't know how to fix it the bad quality.
Many questions on 3D Printing SE are answered, at least by me, not with a &quot;this is the problem&quot;, but instead, this is how I would analyze it.
Looking at the picture you supplied, there is something strangely periodic about the under-extrusion in the top visible flat layer.
I don't think it is an X or Y axis problem.  It is too low-frequency to be an extruder drive problem, such as a slippery drive hobbed gear.  It seems too high frequency to be a filament feed problem, unless you are very close to the end of a spool with a small inner diameter.  This is less likely because you have printed many things, and I would infer that you have seen this with full spools and empty ones.
The Ender 3 looks like a Bowden-fed printer.  You are printing PLA, which should work well with Bowden.
Temperature variation could also cause this problem.  If the nozzle were cooling and heating in an unstable control cycle, where the nozzle is cooler you would find under extrusion and even non-extrusion.  This looks compatible with the results.
What could cause thermal problems?

Not delivering energy to the heater, perhaps because of a bad
connection, or perhaps because of an intermittent heater cartridge
Not sensing the temperature correctly, perhaps because of a shorting thermister
Thermal control look is out of tune, which I consider unlikely since you haven't messed with it, and in my experience that is pretty forgiving
Not conducting heat from the heater to the nozzle, which is unlikely

If you have an infrared thermometer, put a piece of masking tape on the heater (to get a consistent IR emissivity) and monitor the temperature.  Is it cooler when under extruding?  It might only be a few degrees different, but that could be enough if you are right on the transition temperature.
If the temperature is stable, you might try printing 10 degrees hotter, just to see.  Some variation is normal, and you might be on the edge.
It is possible you have a filament that widely varies in diameter, but for that strong an effect, you would have noticed that the filament was thinner in some places than others.
My best suggestion is to look at temperature.
Further Reflection
It is also possible that you have something dragging on the filament spool.  I don't like it as a solution, but it is consistent with printing a lot (at the beginning of spools) and then encountering this problem.  A problem feeding, although I discounted it in my earlier answer, could could produce a periodic underextrusion if the drag on the spool changes.  If the problem was one-time, then it may have been bad winding in the spool so that once per revolution it requires some extra tug to unwind the filament.  That tug would reduce the torque available to press the filament through the working side of the printer.  There might be extruder jumps (pops, clicks, etc.) coming from the extruder as it skipped steps.