feeling discouraged

some people tell me I'm young & still have time. people will probably tell me that here. I'm 25 years old. ever since I was 10 I've wanted to be a mom. I lost my virginity at age 22. so obviously I have been trying to ever since then. 3 years doesn't seem like a long time to be trying but it's been so depressing to me that I haven't. I don't even have normal periods. the doctors can't find anything wrong. they just say "lose weight & we'll go from there." I weigh 285. I have been trying to lose weight but it's been such a struggle. all I think about in my head is "this is for my baby" so I keep trying & trying. boxes & boxes of pregnancy tests wasted. every single time it's a no. every single night it kills me that the only thing I've ever wanted in life, I cant have. my body doesn't want me to have a baby as bad as my heart does. I've always felt like being a mother is my purpose in life so when i lay in my bed every night, I cry that I can't be that. instead i live through other moms & trusted being a baby sitter to everyone in my family. I would like to baby sit my own baby one day. but it's not really looking like that's going to. happen . & my heart Is just sad. I'm just sad.