Ok. Question about loosing weight yet still wanting to get pregnant in the future?

Ok since having my son 3 years ago I am finally trying to loose weight and going on and beyond to loose 20lbs ( back to my 140lbs prepregnancy weight) but just a random thought came to mind. I want to have another baby ( last baby) sometime in 3 years and I’m thinking here wth will all this hard work and effort go to waste whenever I do decide to get pregnant? I mean with my last pregnancy I did try to eat healthy and was mild active but for some reason I stayed at 160lb after pregnancy and its been a B**** to loose it. Any advice? I really want to be at my old weight but I’m over here thinking about the future???😩