She said yes but we can't marry

I had feelings for one of my best friend and probably she had it too. Coincidently we had a talk about it and I accepted that I had feelings since we met. Knowing this, she said yes, somehow convinced by the fact that I will marry her in future. Now she looks in love with me and seems so much comfortable, calls me, text me very often, wants to meet me and all. But I have family problems can't marry her, This happened last week and we have been doing like crazy lovers since then. I don't want to hurt her feelings but I want to say her truth that I can't marry her. What should I do??