I havetohold the handle down longer than usual to flush the toilet. How do I fix this so I can let go of the handle and it will flush immediately

You could install one of those floats that clips to the flapper chain. It helps keep the flapper from closing too fast giving the tank time to empty to completely flush slower flushing toilets. An old fishing bobber will work as well, although you may need to try different sizes until you find what works best.

Could also be the flapper itself though...

Sounds like you need to adjust the length of the chain see the image below, Thanks.

John