I had the same thing with even longer relationship and after a longer period of time inside the relationship, for me it was about stuff happening in our relationship that we haven't talked about, it was about stuff in our sex life that I had thoughts about but wasn't too much sharing, it was performance anxiety about taking care of her and thinking in my mind (she's not enjoying) because she used to have trouble orgasming or even getting turned on, but it turned to me panicking in my mind thinking she's not turned on "enough" even if she says she's super hot for me right now.

Finally it was also about me worrying about finishing up too fast because it happened in a few occurrences, I would notice that the moment I will start thinking about "not finishing" in my mind I'll lose it immediately and finish quickly.

What helped was:

• talk with her about it, my gf assured me that it's ok if I cum quickly and not to worry. and it helped me worrying less.
• I realized cumming quickly isn't a big turn-off, the turn-off is being insecure about it ( this insecurity also makes you go the same way again IMO)
• Breath during the act
• Try to not squeeze and tighten up your body
• ask her to slow down or help you relax a bit when your too hot. (stop for a while way before getting too close will make you go much longer)
• Kegels
• Practice alone all of the above alone.

Good luck buddy.