So, in addition to using lube, lots of foreplay, and doing your absolute best to relax, I'd also suggest trying to stretch a little. One stretch that has worked for me is lying on my back with my legs butterfly style (heels together, knees open). Try to do that stretch every day and see if it starts helping. I'd also recommend bring a vibrator in to help distract you.

I've been dealing with this for quite some time, so feel free to PM me if you want to talk more.