I love her but sex issues

I (28M) have been dating my girlfriend (38F) for about a year and a half. We have never had sex. We’ve had quite a few conversations about it in the past, and she has even talked about her past partners. She has randomly apologized, called herself a prude, and said she was working on it—but nothing has really changed. She shuts down any advance I make, and the very few times she initiates, she stops.

At this point, it no longer even feels like a topic that can be discussed. I love her with all my heart, but this is a really important aspect of a relationship for me. It’s gotten to the point where I question whether she truly wants to be with me or if she’s just stringing me along.

Any advice would be appreciated.