Advice for first-timers

I am planning on backpacking Europe (18 countries) for 3 months starting at the end of May. For those of you who have backpacked before, what is some advice you could suggest for me and my boyfriend? We both have never really traveled outside of visiting back home or resort based destinations. I am very type A, and I know I have a hard time going with the flow - which most likely I will have to get over given the type of trip. Is there any advice you could give on that standpoint as well? Thanks in advance!