I've been wanting to start working out because I'm not happy with my body. Like I'm still tiny, no…

I've been wanting to start working out because I'm not happy with my body. Like I'm still tiny, no stretch marks, etc. But I miss my senior year body. I had a roack hard 6 pack, nice toned butt, etc. Now I get remarks from my boyfriend (not fully directed at me) about how he likes bigger butts, he's made comments about my nose, etc. Even started ordering leggings off of my legging army for me to use to workout since the sizes have a large size range, so if I gain or lost they'd still fit. I just don't know how to gain any motivation! Pic for attention and to show how I currently look.