Unfortunately, your situation is far from unique. Even when parents stay together, one tends to shoulder more of the child-rearing and bear more of the necessary sacrifices. I suspect your ex wasn't an all-in 50-50 parent before you split up. When I split with my (ex)husband, he was all gung-ho on 50-50, and we tried it via temporary orders for 18 months. To give credit to him, he did take his time, but it really didn't work for any of us - especially the kids. I ended up with primary parenting time, he had relatively generous time, but at 300 miles apart, the "burden" fell to me. And I wouldn't have traded it. Sure, there were sacrifices - and thank heavens my parents were close enough to help,
Point being - once you're a parent, you have to go all-in as though you are the ONLY parent. If you're lucky? The other parent feels the same way. Otherwise - Tag, you're IT. And you do what you need to do. Whether it's eating a late dinner of ramen 'cause you can't afford a solid meal for three (with a perpetually starving athlete), making do with the same shoes, jeans, underwear that you've had for five years (*), or yes, postponing your own schooling until the time is right. This is the part of parenting that they don't always tell you when you take the job.
(*) Giving him yet more credit (who here ever thought?), my ex was faithful in paying CS, but a 3hr daily commute to keep my "well paying" job wasn't in the cards - not if I also wanted to BE a Mom...
p.s. Wait until he gets a new g/f who's less than enthused about sharing his time off with his/your "brat"... LOL Let alone the money he's ordered to support her.