It sounds like there are 2 things going on here.
The first is the lack of freedom to do things spur of the moment. I actually think this is quite normal, particularly if you don't have a partner who is in most evenings and weekends so you have to pretty much double-plan everything you want to do. It means you do a lot of solo parenting. It's difficult.
I get what minipie is saying about changing things but it's not that easy. We do it, both working f/t and doing different shifts, and it's fucking hard sometimes. I can get very resentful of other friends who can just do things at the weekend, or go out in the evening, because there OH works sociable hours. I really do understand what you are talking about there. Every now and again it gets to me and I feel rubbish for a while because it's hard work.
BUT it very much does seem to tie in with your second problem, your fiance.
I can say, working from a very similar set up to you, that while I find it hard to have very little time where I'm neither working nor being mum, while I find it hard that I don't see my OH as much as I want to - I don't mind, because I am happy with him, so we make it work.
It sounds like you aren't in love with him any more. You say you're "not happy". Coming from a similar perspective, I can say that isn't how I feel about my OH purely because of the hours and the difficulties of tag-team parenting. It sounds, very simply, that you aren't in love with him any more.
I agree with some of the comments above that as your son gets older you will gradually start to get a little time back. I'm lazy; if I have DD alone all weekend then one day she gets a film on and I just lie on the sofa with her a read a book, rather than doing housework. Also at nearing 3 she is old enough to go off and explore on her own at soft play, and at 9am on a Saturday morning ours is just quiet enough to sit and enjoy a coffee alone. It's not much, but you have to grab what you can! And then as he gets even older, there will be playdates with friends to consider where you might be able to leave him for a few hours.
It sounds like you need a good bit of "me" time very soon to try and start sorting yourself out. If your OH works at weekends, does that mean he's off during the week? Can you book a day off work and go and visit a friend because your OH can look after your DS for two days? Or as your mum or MIL to have an overnight so you can get some time alone?