Is this sexual assault?

About 2½ months ago, I met my best friend, who is a male, to say goodbye before I left town. We liked eachother romantically, but weren't in for a relationship at the time. The day I met him, we kissed a couple of times and it was nice. We were sitting in the grass of a wooded area and he started pulling my pants off which I was NOT down for. Everytime I tried to pull them back up, he'd tug harder. Once he got them down, he threw my legs up over my head and I said "stop, this isn't going to work" and he continued. I tried to push his head away and close my legs, but he would just push them open wider. I told him to stop a few more times, but he wouldn't. Once he was done, he asked me if I was done and I said yes and that I had to go home. I feel like I could've done more to stop him. I feel like it's my fault