A heavy bike (weight) will cost more money to make lighter than it would cost to buy a modern bike.
You don't say what type of bike you have, so answers can only be very general. I have very old bikes with hub gears and they are certainly not heavy.
A £70 mountain bike will always be heavy, and the gears will be 'troublesome'.
If the gears don't work correctly I would suggest that you take it to your local bike shop for repair and have a look at some so called 'decent' bikes and have a look at the prices.
Unless you are riding specifically off road, you will be much better off with a road bike. Maybe one of those modern road bikes with flat handle bars and disc brakes.1 month ago