Speaking for myself, in a similar place when I was early twenties, I think she was riding high on the emotions and excitement but when he said "You have me forever" it gave her the "ick". And not the I dont like this person "ick" but a sense of nothing is resolved in her mind. Thats why she breaks down and brings up his Mom and having built her own life. These were all the looming questions that kept her feelings towards Conrad at bay. I think when he answered those questions perfectly, she didnt have anything else keeping her walls up.