Hi S.,
I guess you can see by now that you are very much not alone. Sometimes I feel like it's maternal hormones gone awry! I'm a single Mom to one, and I tend to isolate myself. I always feel more balanced when I get out and see friends, with or without the little guy. When your life has a balance of being all the different things you are, you may be able to get more perspective. For me, I need more companionship (I try not to use it only to discuss my worries about my child), for you it may be something different. A hobby you dropped, reading a book or a favorite TV show. I love going to the movies; the show, the popcorn, the pals. Now, when I do go, I feel like I'm on vacation, it's exhilarating.
Your son is still very young. Don't berate yourself if you can not get to the gym right now. Yes, excercise would help, the benefits are endless! Eating right too. Just don't be too hard on yourself, an 18 month-old is a very demanding little creature.
Of course, a therapist is a great way to get perspective. It's hard to find time to see a friend or a therapist when you work full time and care for a child.
You will get yourself back, it takes time.
All the best,
C.