Lots of founders go through this. You should have an honest conversation with your co-founder. Probably couples therapy. One option is to do it with another founder who has been there (me).

You both are going through a challenging time. Of-course you are going to find some things that are more interesting to you than your founder, and vice versa. Especially if you are pivoting every month (which is a whole different problem).

Going through tough times can build resentment, the way to get on the same page is to talk.