I’m only 17 and my parents got divorced when I was 5 , so for ~ 12 years I’ve seen my father an average of once a month, although it’s frequently extended past this. I’d consider myself incredibly fortunate in this regard since many children see one of their parents much less often that this if at all. However, if my dad was able to have more frequent visitations then I would hope to god that he would prioritize that over his girlfriend.
If you don’t understand how this development in custody is a big deal and it’s great for both him and his children then , quite frankly , you’re selfish for being in this relationship. I understand that change can be scary , but if you’re mature then you’ll put aside your fear and try to be supportive. If you don’t feel ready for things such as this , then that is okay and perfectly reasonable , but you need to be honest.
Three months really isn’t that long , so if you think that you just need some time to get to know this person better and jump into a new routine, then that’s fine, but if you think you need to leave so that you aren’t in a situation where you might feel bothered by him needing to adapt his schedule (which might be less stable than a strict every other week rotation depending on circumstances) then decide quickly and have a conversation with your partner.
It’s okay to not feel certain about things. Change is often difficult and you aren’t terrible for thinking about this or being scared or just not knowing how to react. If you aren’t ready to be in a relationship where kids are involved then be honest with yourself and the people around you. It isn’t fair to anyone to keep entering relationships where things might be complicated when you aren’t ignorant of the circumstances. Just take some time to breathe and think and either stay and adapt and be supportive and don’t force the situation or be honest and leave and take some time to just be with yourself so you can figure out what you actually want.