Feeling defeated

I'm only in my fourth month of trying but I feel like a failure. My period is 5 days late and I got a bfn. My mind is making me feel pregnant but I know that it's unfortunately probably not true. I can't help but keep comparing myself to my sister in law who got pregnant the second month and said it was so easy to do so. I just feel like a failure and that I can't do what I want to do. My entire life I've set out goals and completed them and I just feel like I can't reach this goal. Thanks for listening to my rant.