firstly my dd is 5, I have been on my own through out like your self. I found 18 months a very difficult age and particularly difficult alone. It is an age when the child is staming out and demanding indipendace but also require and demand your attention....you have less controle over the childs movement and activities to your own convenience like you do with a much smaller babie and everything seems to need negotiation and reasoning and quiet frankly the child does not pocess those skills.
Sounds like you are and have been doing a great job. I would not consider antidepressants, but you could always go have a chat with gp.
I felt exactly how you are and remember feeling I may lose my rag one day and never actually calm down, I remember once actually walking out of room and locking myself in the toilet because I though If I smack this child I may not actually stop. No sooner had I turned the lock clarity hit me and I felt shamefull and guilty. dd was distressed and tired and so was I but we moved on from this and came out the other side, things that helped me.
discipline your child for unacceptable behaviour, I used at that age three strikes and your out. tell your child, no do not do that, if continues, tell child, if you do that again mummy will sit you by your self/or get you out bath for instace....if they do it again sit them out of the room/ or get out of bath.
this realy helps take you take control and ends that constant feeling of tolerating and tolerating untill you finally blow a gasket. and if they scream, kick and melt down, its not at your feet, there is distance between you both so you can gather your thoughts. rationalise, something has to give an that cant be your sanity or fuse.
adjust routine for the tasks that are more time consuming or change ommit when both tired. ie long day at work and nursery, no bath tonight, snuggle in bed earlier.
teach your child to amuse themselfs, set up an activity and tell your child you will play and then do xyz, become less involved gradualy and slink off to do xyz. belive me this gets easier, my dd was so clingy she had to be touching me all the time by 2 she would play alone for long enough for me to wash dress make up etc in the mornings.
pick your battles and set routines, you are only human dont put to much preasure on your self.
is there anyone, a friend or family member you can just pick up the phone to from time to time. I feel for you but belive me it gets easier.