please help me

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At the time I am in a very complicated relationship. We have been together for almost 3 years. He is 2 years younger than me. Somehow we are really happy. He can be caring an funny and I really like him but then he can get really angry and he is in full rage mode and then he is cussing and calling me names and telling me how useless I am... and those words can hurt so much. And sometimes it even gets physical. He slapped me more than once, sometimes he grabbes me really hard or pushes me away.
But that only happens once a month or so. I don't consider it that harmful it only is.... I wish he would learn to control his anger and I asked him a thousand times to go to thearpy, he already knows that he has problems, but he has no time for a therapy and clearly doesn't want to work on himself.
But after each fight we get things together again and everythings fine. We kiss each other and he apologizes deeply but often doesn't even realize how much he hurt me (physically but mostly verbally)

I really don't know how to cope with this situation. We are engaged now and he wants to marry soon and somehow I have everything in him that I need but that anger problem.