It'll break his heart

2014 I had a miscarriage and my boyfriend was so heart broken.. Once I had the d&c we wanted to try again. I was so scared to miscarry again so I chickened out and been on birth control for about a year now but he's had baby fever ever since.. In Janurary we talked about wanting to try again but after a few negative pregnancy test I've had time to think to myself how I'm not ready to be a mom just yet.. I haven't talked about it to him because I don't know how to tell him and I'm not ready to break his heart again.. I know this may sound so selfish but I'm so self conscience about my body, I weigh 182 and have stretch marks. I don't want to get more stretch marks and gain more weight.. I want to focus on myself right now and try to lose weight and finish school.  Neither one of us have our parents in our lives so it'd just be us.  We've been together for 5 years and we're moving in together in 5 months, but my sisters will also be moving in because I want to get them out of dcs custody. I think it would be so hard taking care of 2, 16 year olds and a baby as soon as I move out. I feel like I'm being selfish by putting my body and feelings before him again.. I really need some advice.. How should i tell him how he'll understand ?