How do I reject her in the nicest way possible?

We (both early 20's) matched on a dating app a while back and went out on a first date two days ago.

Needless to say, she looked much different IRL and was somewhat heavier than her pictures suggested and I felt absolutely zero physical attraction to her whatsoever. She seemed to be quite attracted to me though.

In spite of this I did try to keep the date fun, and our conversation was very laid back and in tune. We vibed pretty damn well, and she opened up to me about her bad issues with mental illness and struggle with self esteem. I had been the first guy she had ever gone on a date with, which kind of shook me.

Now I feel terrible and shallow, because I know that if I tell her that I found her unattractive she will feel terrible. How do I reject her in a way that makes sense while not offending her? She is a genuinely nice person and I do not have it in me to ghost someone who went out of their way to give me a chance, even If I do not find them attractive.

I need advice, thanks.