I would want to tell myself that I wasn't really in love with the guy I thought I was in love with and that he was nothing but toxic.
To suck it up and seek help for my anxiety and grief because I suffered for years stubbornly trying to get over it on my own.
And to not rush into things.
But really, all of those things got to me where I am today and I think I have it pretty good. If not for those mistakes who knows where I'd be.
So I guess REALLY what I'd say is to not be too hard on myself, that I'm going to make bad choices and get hurt, but that it will all work out in the end if I own it and learn from it. I would tell myself that now at 27, too, because I'm sure there are more mistakes and big changes yet to come. :)