I feel crazy

So this is a long story and I'll probably ramble, but I'll try my best to keep it short.

So my fiancé and I have been together going on 5 years, were supposed to get married next summer.

So back in April we moved into this house we were lucky to find because its a short 5 year contract rent to own modular. Everything is going great. Fast forward to the beginning of August and this chick asks my fiancé to help her redo her roof. She lives down the road from us and saw him on our deck and decided to ask him. He told her he had never done roofing especially metal roofing. She says she doesn't care that it will work she just needs someone to help her do it. Mind you my fiancé has a background in construction (mostly concrete buildings) and at first I didn't think much of it. So then a few days go by and its time for him to help her, I didn't care just told him to be careful. Then I noticed him slowly starting to brag to me about how cool she is and all this shit, he finally noticed he was starting to piss me off and then compares the two of us and how we're so similar. As the days go by she starts coming to our house more and more, and i notice that if I go out onto the deck to smoke(yes I know smoking is bad, im trying to quit. Anyways we only smoke outside) if I go out to have a smoke and rheir out there talking she glares at me and for the most part completely stops talking. This is when I realize that she fucking likes him. So I tell him yo I'm not cool with this she has the hots for you, finish what you told her you would do and dont do anything else for her please because im not comfortable with this. A few days go by ans I find out that after he agreed to not do anything else that hes agreed to help her do more shit. Then my uncle dies unexpectedly and im an emotional wreck. Be stays away from her for a few days. Then he accidentally lets it slip that he has her cell number. Fast forward through more bullshit, and last week this bitch decides to bend over the railing on our deck and shake her ass infront of him. Mind you I'm in our livingroom where one of our monitors for the security camera system is and I'm watching her do this. He comes in thw house and tells me theres another thing she asked him to do and he said yes. So I flat out tell him if youre going to continue to be her little fix it guy lap dog that im going to lose my shit. And i told him that the next time I ever see her pull crap like what she had just done on the deck that I would grab her by her hair and she'd learn how quickly you can be thrown from a deck. A few days later be asks me to text her and say that he can't help her the next day. This is when I realize hes been deleting all their messages, the only messages visible were from the day before. They have been texting for weeks and he deleted the messages. This man never deletes messages ever, he has texts from 2015 from wrong numbers . mind you I never ever go in his phone unless he asks me to do something on it. I have never ever been jealous of another woman while in our relationship til now. I literally have been so sickened by this that I've been nauseous every day. I'm considering just walking away.