When I'm on say 50 Km/h my first gear is not accesible and second gear is too, 3rd gear however is hard to push in but 4th gear is easy to slip in, why?
I'm assuming you have a manual transmission.
Stop doing that, you will damage your transmission.
Your powertrain can be broke down into three sections. The engine, including the fly wheels. The clutch and input to the transmission. Finally the transmission output.
When you push the clutch the engine is disconnected from the transmission input. When you shift a gear, you change the ratio between the input of the transmission and the out put. Also when you shift gears there are collars called blocking rings that slide over and engage a set of teeth to lock you into gear. The blocking ring and teeth have to be spinning at the same speed for this to happen otherwise they'll just grind. To get them spinning the same speed a synchronize is used. The synchronizer uses friction to get the two part spinning together. As you drive you accelerate the engine to gain more speed. Then you push in the clutch the disengage the transmission. As you shift into the next gear the synchronizer will slow the clutch allowing the next gear to engage. As you shift gears the engine also slows so that when the clutch is let out they are roughly at the same speed.
Now lets play a hypothetical game. Lets say you can reach 20Km/h in first gear at 6000 rpm. How fast would the engine have to spin to get the car to 40Hm/h. The answer is 12000 RPM. How fast would the engine have to spin would to get the car up to 60Km/h. the answer is 18000 RPM.
In this hypothetical if you're driving at 60Km/h and try to shift the car into 1st gear how fast are you asking clutch to spin. The answer is 18000 RPM. The synchronize just can't handle that. Because there is such a difference in speed the gear just won't engage. It's probably grinding but so fast that you can't tell.