I couldn't fit inside her, what do I need to do?

I have been together with my amazing girlfriend for nearly a year now and last night we finally decided to have sex. It was going to be both our first times, so we dint really know what to expect. It started with me doing lots of foreplay with her and getting her super turned on and wet, yet even with my fingers I felt how extremely tight she was, It was difficult to even fit one finger inside her, and after roughly an hour I manged to loosen her up enough to get 2 fingers in which was still a tight fit... Anyways we continued to kiss and I gentle talked to her to relax her and really tried my best to get her ready, so after probably an hour and half of foreplay and kissing we decided it was time. Now before I continue, we talked a lot about this before hand and I knew she was very tight already, and she knew I had a large penis (about 7.5 in x 5.5in if that helps you answer this question) but I never expected it to be this hard... I tried to push myself in gently while kissing her and making her feel loved and relaxed, but I swear I couldn't even get the whole tip in, It was like a wall, I couldn't even feel that there was an opening, and I slowly pushed harder and harder, hoping to slide in, but it was starting to hurt her... So I went back to foreplay and we continued this loop for nearly 4 hours, even going into the shower together with warm water, all without any success. We still had a good time and I made her get off with my fingers and I jerked off to finish. What are we doing wrong? We both want it so bad but despite al efforts I can't fit my penis past the first inch inside of her. She even broke down crying thinking it was her fault and it broke my heart that she thought that and I had to comfort her and reassure her that it wasn't. The one thing I think we definitely should have done is use lube. I though I had some with me but I guess I forgot it somehow, but for any of you out there, was are some other tips or advice you might have?
I couldn't fit inside her, what do I need to do?
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