If you want to change it just a little bit, probably you can make these changes:

So, the final sentence would become this:

You should buy it now, otherwise you're going to waste your time thinking about it.

But honestly, even then it isn't really natural sounding to me. Most likely what a native speaker would say is something like this:

You should just buy it! If you don't, you'll regret it.

I'm from the US though... it might be a little different in the UK.

英語頑張ってくださいね！