Ok so i really need some advice

Me and my husband have been together for 5 years and we have 3 children together. The last 3 years of our marriage our sex life has been horrible I am lucky if we have sex twice in 5 months. I have tried talking to him because I have needs to and he will promise to work on it, and it will get better for a week then right back to this. I have recently met someone who I have gotten to know really well and I truly am starting to think about sleeping with him. Please help me figure out how to fix things with my husband before i make a mistake!