Arguing over money

Long time poster, but NC.

DP and I live together and in general things have always worked out fine money wise. We earn the same and no children.

Money is the constant cause of arguments though and it's getting me down.

We got Splitwise for things but in my mind it was more to ensure things naturally evened out over time. We own a house together, we are buying a new house and are getting married...

So I now if we don't sort this out now it could end badly.

I put a holiday we went on on a 0% credit card and said DP can pay me back over time so the card is paid off. I've calculated my excess money to pay this card off just in time for my share. I can't afford DPs share. He has £500 left to pay. He usually is good with this but has come out with "when you pay me back for the stuff on splitwise" the balance was £89. I found this ludicrous as splitwise has menial things like someone bought the big meal out and puts it on there. I never put much on there as I just can't be bothered. I pay most the house food shops on my personal account which will have his things on it too, as he does the same so I assume it equals out.

But now I am just angry. I think my anger comes from a few things:

1. I wanted to stop seeing it as yours and my money - we are getting married now so it should become shared. If not:
   
2. why am I contributing more to next house just because I saved more money? He wants the house just as much as me.
   
3. how will this work when I have children? Are we going to argue over money.
   
   I am angry and frustrated. This argument has probably stemmed from me asking him to put his washing away. He's so lazy and thinks he isn't.
   I want to cry.