If I am already living together, how difficult will it be to draw up a cohabitation agreement? Will that be even possible since we already obtained things together?
No. You can draw up a cohabitation contract at any time. It is not like a prenuptial agreement that has to be entered into prior to the wedding ceremony. You can contract right now as to what happens if you should decide to part as far as property purchased together and divide up who gets what.
No, it's not too late. I'm sure you still remember who brought what to the relationship, and now all you have to do is decide how to treat the things you boguht since you moved in together, and how you'll treat the things you buy in the future.
Your lists of possessions might be a little longer than they would have been had you done this before moving in, but it won't be that much more complicated. And since you're already living together, you both have a good idea of what doesn't need tweaking and what does.