Am I the worst mother?

I don’t know if this is the correct place to say it, please don’t all shoot me down but I feel totally worn down and surely I cannot be the only one?! I have 3 wonderful children the youngest (6) with a rare type of CMD, my youngest uses a power chair, is tube fed and has very little upper body strength. I’m continually trying to balance the needs of all my children combined with the extra needs of my youngest which includes tube feeding, toileting, continual repositioning, trying to come up with activities so he is not bored, trying to help him cope emotionally, dealing with professionals, continual broken nights sleep – we were in hospital one day last week, we are back in hospital all day tomorrow, school is going pear shaped and requiring loads of extra meetings, my son is now losing his ability to write and is having to start using a voice recognition programme. My husband is working long and hard hours, I’m trying to hold down a part time job during school hours for a bit of extra money and now I have a prolapsed disc in my back. I probably sound like a nasty ungrateful mother but if I don’t tell someone I feel like I will explode! I love my husband and my children more than anything in the world but right now I am exhausted and feel like I’m drowning, there must be someone else out there who feels like me and came out the other side??!