Limit drawn current but not voltage (LED driver for 100W LED)
We'll be able to help. Firstly, here are some guidelines in the right direction. Look through these and 'report back'.

Here is why to use constant current

Here are many possible ideas - look at these to get a general idea and then we can discuss in detail.

You can also buy drivers ready made. Here are some examples - doing it yourself probably costs slightly more but means you can tailor it to just what you want.