How do I deal with this constant accusation?

My Fiance is convinced that I am cheating on him and it has gotten so bad that the things he imagines just doesn’t make any sense to any normal Human being. He has gone as far as accusing me of SLEEPING(to put it in a nice way)brother, friend, neighbor, a man at Walmart, basically any male. It has gotten soo out of hand that he thinks I get up and night to go do a man and come back to bed. Or one time he was upstairs for not more than 5 minutes I was downstairs cooking and when he came downstairs he says that I had just did my thing and let him out the back door. I don’t know if he just has a jealousy problem. I believe he has more of a serious problem than that. More like Paranoid Schizophrenia. he’ll say he hear things and see thing and believe people are out there to get him or hurt him including me. Where ever we go what ever we do if there is any male figure with our vacinity it’s a problem, he’ll start his crazyness with me wanting to be seen or wanting to “F” them, and keep this in mind that I’m not even paying attention to all this that he see’s. This is just a rated G explanation, it’s worst and could go on forever. And I’ve given this man everything. He does this every other day if not every day. I’m 39 years old with 4 kids and a grandbaby do I really have the time to go through all that trouble just for a piece of “D” when I have one? Can’t he realize that? Should I believe that he is capable of loving me?