I agree with Sam's mum. I have actually been through a similar experience. The father of my first 2 children left me when the youngest was 6 mths. I also had a 19mth old. Over the years the children have has to cope with various different situations, where they have lived with me full time, and also with both of us for many varied times. I have experienced lots of clingyness, complete defiance, bed-wetting, "but dad lets me do...", "I don't love you any more", and every thing in between. I have found that any thing is normal for this kind of situation. However, if you think you have a real problem, or see that a real problem is beginning to develop, then talk to a professional about it, whether that be your family doctor, child health clinic, or just anyone you trust.
If you wish to talk more, feel free to e-mail me at email@example.com
All the best
Lyndal, ACT, 6yo boy, 5yo girl, 8wk boy