I’m sorry to hear it, OP, it sounds hard. Your boss is out of order asking you to do extra shifts, but you are within your rights to refuse - can you say that you can’t leave your children?
Personally in your situation, I would head to the doctor and see what they say. If you’re in a real state you might get signed off work for a few days. Regather yourself, look into finding a new job, or at least spend some time steeling yourself to stand firm on your work boundaries. If you don't want to go to the doctors, you could look into some cognitive behavioural techniques that might work against sliding into depression. One technique is for every negative thought that you have, force yourself to think about three positives of the situation this reshape your brain into viewing things more positively.
For example, re work you could tell yourself - it's good that I am needed at work - better busy than not, it's good that I have a job rather than being unemployed at the moment, it's good that I have a job while looking for a new job because often it's easier to find a new job when you're already employed. Obviously some of this will feel like you're bullshitting yourself a bit, but the idea is to help your brain to see the positives, so the over time your brain becomes more positive!
Re the guy who might be trying it on the positive might be - I am attractive to men, it's an easy rebuff since he has a partner, I figured out his game already so I can cut it off early - at least I'm getting better at protecting myself. Again, the point of this is to give your brain different things to think instead of thinking I can't cope with men trying it on, why does this always happen, et cetera et cetera - the downward spiral of negative thinking that can lead to depression.
You did the right thing ditching the ex - ignore the PP (horrible comment!), you should feel good about making that decision for yourself and your children. But it sounds like you could do with realising that you are worth more than you seem to think, to give you the confidence to continue to set boundaries that protect you from being exploited at work and from entering into unhealthy relationships. You are worth it!
I would spend some time thinking about all the good things that you have done, all the things that you like about yourself and remember that this is who you are, and that things will get better.
Big hugs xxx
edited to remove typos