Find it hard to put on weight?

Have you tried putting on some weight but your attempts have been thwarted by your ‘fast metabolism’.

It’s not your metabolism! You are likely a very active person; lots of exercise, lots of moving around or a combination of both.

If you are looking to put on some weight and build some muscle then you are
going to need to increase your intake to a point where you are consuming more than you are expending. Only then will weight gain be possible.

To ensure that the weight is going on as muscle [as much as possible] you will need to perform adequate resistance/strength training. Also you need to be consuming adequate protein.