I hate being a stepmom and want to give up

I have 2 biological sons (6 and 11) and 2 stepdaughters (6 and 8). My boys dad and the girls mom both passed away so its just me and my partner parenting all 4. Ive been their full time mom for a year and a half now and nothing is getting better. Honestly its getting worse. I held on to hope that if i were just patient we’d hit our stride but mothering them has just caused me to feel like i hate being a mother. I never felt that way with just my boys. Of course we had our bad days but they were far less frequent and not enough to make me question what i am even doing with my life. I dont enjoy their company, i dont enjoy doing things with them, i do way less stuff with my boys now because i just cant stand taking the girls to do anything. Everything is a problem, something hurts, theyre tired, theyre scared, the list goes on as to why going anywhere is just not worth even trying. Me and the boys would go on hikes, go to the park, go out to eat, go to friends houses and everything was great. Now when i try to do things with the girls too its a huge fiasco and theyre ready to go he 20 minutes in. Id do projects with my boys and theyd make an effort even if they thought they were bad at it. The girls break down in tears and wont even try and it kind of brings down everyones enthusiasm when theres someone sitting there sobbing or storming out of the room and crying loudly from their room. Pranks make them cry. Jokes make them cry. If they see anyone else get anything at all they whine about ‘how come they got one and i didnt?’ Instead of just asking or god forbid being happy for someone else. I understand they have been through a lot but so have my boys and its like life has to completed hault to accommodate the girls. I barely do anything fun with them anymore because i am so overwhelmed by all of the attempts that have gone all to hell. I want to end things with their dad (whom i love) just to get back to the life i was building with my boys. I know it wasnt perfect but i miss seeing plans through. I have brought this up numerous times to my partner but he says im just trying to be lazier and do less. He works and i dont because we have 1 vehicle and i drive him to and from work (3 hrs driving daily) and am the on call parent so if any kids have appts or are sick he doesnt have to miss work, im there handling it. I couldnt possibly hold down a job with as much as the kids stay home for 1 reason or another. Theyve already missed so much school and his youngest was coughing tonight and he was kind of insinuating maybe she should stay home and i lost it tonight and spilled ALL of this in 1 shot. I cant even begin feeling like im living my own life and all he has to do is say the word and his life goes on as normal but i have to be ms. Flexible and forget anything id planned on doing. Im just so mentally and emotionally exhausted and ready to call it quits. Like even if that makes me the bad guy im at the point id rather just live with that knowledge and move on with my life. Lesson learned, i am not step mom material and if that means i am single forever then so be it. I hate being a mother now. Feeling like im failing all 4 kids has really taken its toll. Has anyone else gone through this? What do i do?