What Should I Do?

I am 18 years old and in love with a 20 year old woman. We have been flirting for a few weeks by her constant joking that all is my fault as man and I respond by telling her she's a woman and it is her fault plus a few comments about her being blonde the thing is we are both laughing the whole time. She is constantly smiling at me and on several occassions grabbed my arm saying she wanted me to see how cold her hand was. We have talked about intimate things such as how many and the names of the children she wants to have she has also, complained to me about her boyfriend. About a month ago she told me she was not ready to get married and now on Monday I hear her and her 32 year old boyfriend are engaged. I love her and she is making a mistake, should I tell her how I feel?