Sound to me like it's your job to take care of everyone else, and no one is taking care of you. You aren't exactly a new mom, but you do have a little one who is constantly changing, so I'm sure every phase seems like it's all new to you. I am sorry about your fiancé and his past/present medical issues, but if I understand you correctly, he's not working, paying bills, cooking, cleaning, shopping, providing childcare for the child you have together (either entertainment or making bottles or anything else), or doing anything for the stepson. (Not sure why you have HIS stepson - do you mean it's YOUR stepson and his son? If it's not his child, why is he with you every single weekend especially if your fiancé is unable to do anything?) I think you should consider revisiting this visitation arrangement - the child is not getting much from a father (or father-figure) who is unable to do anything at all, and you have turned into the babysitter so that child's other parent(s) can have some time off. Unless this is court-ordered visitation, I'd cut back on that, and I'd make sure my fiancé was stepping up to the plate on it!
And if you get a babysitter, it's not for YOU time, it's just to make the other work more efficient (like shopping without a little one in tow). That needs to change.
I guess I would ask you to think about whether your fiancé is getting the medical care that he needs - some people with depression are disabled and exhausted, and others are taking medication and getting therapy so that they can function well, including having a job, contributing to the family, doing basic rewarding things like child care and enjoyment. Is your fiancé so disabled that he is unable to do that? Is that why you are caring for his 2 children with no help? Is sitting around actually helping him feel better about himself? Or would he feel better if he did something constructive? If he has a child already, is he not a more experienced parent than you are, and maybe he could help? Or did his addictions get in the way of that, and he's totally inexperienced?
If I were you, I would use that babysitter NOT to go grocery shopping. I'd let the house go without cleaning every other week - just throw some stuff in a bin in a closet or in the laundry room, and let it go. Don't make the beds, don't clean the bathtubs. Everyone will survive, and you will have to make do with less of a sense of order.
Instead, I would use those 2 hours to get in a 1 hour counseling session (plus some driving or quiet time even in the car!) either with a therapist to help you understand more about depression and addiction and your own need to be (perhaps) codependent here, or to get a life coach to help you formulate some goals and priorities. It sounds to me like you are so exhausted that you cannot think straight, and you are flying from one task to another.
As the flight attendant says, "Put the oxygen mask on yourself first, and then help those around you." I think you are getting zero oxygen, and everyone else is sucking it out of you.