Surgery, jelqing, a penis pump or losing some weight are some options. I don't recommend surgery. If you have the time and patience you could do some jelqing exercises, google it if you don't know what it is.
If you want to stop cumming that fast, you need to mastrubate till the point that you have to cum, just stop before cumming. After that wait until you aren't fully erected and then start all over again. After a few times you can cum. This will help you holding your cum in longer. You could also just cum before having sex, so the second time you are going to cum it will take much more time till you cum. By the way a 5.5 inch penis is average, an average penis is between 5.5 inches and 7 inches.