Getting married, how to split expenses?

Getting married, how to split expenses?
Added (1). My boyfriend and I are considering marriage. 90% of the time we are together, I have to pay for both of us. He spends 3 times more than I do on groceries because he tends to choose more expensive products.

He once stayed at my place for an extended period, and I found myself spending more than I was earning:-(

Now that we are considering marriage, I have bought a house, so I am paying water, gas, electricity, insurance, mortgage (around USD 1300 per month).
Added (2). He has assured me he often gives his mother money for groceries, but whenever he is with me, he is broke (he has a job but is contemplating going back to school, so he would get USD1100 as an apprentice). Still, my typical monthly grocery bill is around USD200.

I want to find a compromise, making sure I am not the one paying for everything…
Added (3). Extra info: He is in his mid-30s!
Added (4). It is the third time he goes back to school. The first time he eventually finished his education, but got tired of the job. The second time he never bothered to finish his education. Now he is complaining because all that is left is a factory job.

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