I am happily married to my guy, but I like one girl. I just want to tell that girl what I feel about her. I don't have her contact number except I know she is on Instagram. She is married too. Should I tell her about my feelings? How?

We used to go to the same gym for almost 2 years. But we never had single talk. It’s been 18 months I haven’t seen her. I am missing her a lot. I just want to be her friend. I can’t lose her. I wanted to tell her about my feelings for her. I feel like we had something, the way she used to look at me. I don’t have her any contacts. All I know is, she is using Instagram but she doesn’t post anything on her feed. I sent her request but it been pending from 8 months. I don’t want to ruin her or mine married life. I just want to know if she feels same for me? Should I tell her my feelings? Please someone suggest me how? Thanks a lot in advance.