I saw a preview where Picard & Crusher were lying on the ground, Picard saying, "I love you, Beverly. I've always loved you." What episode/film is this from?
The closest any episode comes to exploring the Crusher-Picard love interest was the episode Attached. Picard and Crusher are captured and neurally linked. When Crusher picks up on Picard's thoughts, he admits he had feelings for her and, after an awkward conversation, they lie next to each other to rest. He never says he "always loved her", but states that their relationship has changed to something more platonic over time. Emphasis mine
CRUSHER: I remember when Jack and I took Wesley on his first camping trip to Balfour Lake. Wesley kept throwing manta leaves in the fire, watching them pop. Jack kept telling him. What? Jean-Luc, I heard you. Don't push it away. When I said Jack and I, I felt this sudden wave of something. I didn't know you felt that way.
PICARD: Didn't you?
CRUSHER: I guess I always knew that there was an attraction between us right from the start, but I never knew how strongly you felt. Why didn't you ever tell me you were in love with me?
PICARD: You were married to my best friend. At first I thought it was harmless infatuation. Something hormonal rather than emotional.
CRUSHER: Then when the months went by and the three of us began spending more time together.
PICARD: I realised that it was something else, and it wasn't right. But although I would never act on it, I couldn't help the way I felt.
CRUSHER: And when Jack died you felt guilty.
PICARD: I felt guilty before he died. Having feelings like that for my best friend's wife. And then later, after the accident, I promised myself that I would never tell you how I felt. It would be like betraying my friend
CRUSHER: That's why you didn't want me on the Enterprise seven years ago.
PICARD: I didn't know how I would react. And then, little by little, I realised that I didn't have those feelings any more. Twenty years is, after all, a long time.
CRUSHER: And now we're friends.
PICARD: Yes, friends.
CRUSHER: Well, we still have a lot of ground to cover tomorrow. We should get some sleep.
(Beverly lies down near the fire, then Picard lies next to her)
I should note All Good Things..., the series finale, hinted at a rekindling, but that was an alternate timeline and does not match the description