We can probably all say this has happened to us at least once in our dating lives.
For me, this was my second ex. I should have seen the red flags very early on but didn't until they were hardcore 'in my face' and by then the relationship was shot to pieces and irrecoverable.
Note that I've never been married, or wanted to get married, or proposed marriage (to either my first or my second ex). I've never been that silly. 8-) I'm introverted, neuro-divergent, and have traits of ASD so the conditioned linking between 'romantic love' and 'marriage' doesn't exist in me.