Welcome to the forum and for sharing how you are feeling, I think your username is really a great summary of how things are for you right now. I am so proud of you for reaching out and getting some support here and for doing just that..keeping on swimming.
We are going through some really interesting times and unlike anything we have actually had to experience before, and hopefully do not have to experience again. We are not used to being so isolated and it is so much extra pressure not to only keep ourselves in high spirits but our children too, and ensure they are not emotionally scared from this time. You have the extra pressure of also ensuring your step children are ok too, that is ALOT, as you said.
I am hoping that your husband is helping you with this load and providing you with support? If not he may need some direction from you as to what you need at this time, do you think a conversation with him to let him know how you are feeling, that he actually has a role to in providing emotional support to all the family and you really need him to step up more that usual at this time? He may not know how or what you need at this time or that you are infact struggling, so a chat might just bring his awareness to this.
I think it is also a good time to delegate some jobs around the house to the kids, things that they may not have done before, like taking it in turns to cook the family meal, it engages them, it makes them feel valued and if it flops..who cares..but if it doesn't awesome and they will feel so very proud. Also some other jobs like vacuuming or mopping, to also give you a hand around the house, once again it may not be to your standard but it will keep them busy and encourage them to contribute.
I am so glad you do have the support of your GP and maybe a call to check in with him/her next week if you feel like you need to chat and just get a check to see you are ok and if you do need some extra support.
I hope that you are feeling supported here and I hope to chat to you some more.