How to politely reject someone?

That's my plan too! I'm not dating at all, and haven't been for a while. This is MY journey, not about to complicate it by adding a man to the mix. And you'll get there, the confidence thing. I still have a ways to go with it too. Everyday it gets a little better. 

Thank you, I hope you are right and I admire your confidence.  At the moment I feel like a teenager again - all awkward in my body and hormonal.  One step at a time eh? You are so right, it does get better every day.

This is a great time to try on being assertive and work out what it feels like to be a strong woman that gets what she wants, and doesn't feel bad for politely turning down whatever she doesn't want. If he looked dejected, I will almost guarantee he was a bit needy and wouldn't have made great boyfriend material right now anyway. 

I never had a tough time attracting male attention at any weight, and am married to the epitome of "tall, dark, and handsome", and actually turned him down after our first date - but he persisted in a non creepy way and I eventually fell for his charm. ;) 

I used to sport an engagement ring (a cheapie) when I went out when I didn't want to date. It worked like a charm. 

I have a bit of a different take on this. You don't owe this guy anything. You don't owe him politeness or an explanation or an "easy let down".

Let's look at what happened.  He's a complete stranger who comes up to you on the street and asks for a date. You say no thank you.  Then he pushes further, going on and on about how he's so lonely in Ottawa, etc.? Everything you're feeling right now, feeling "so bad" for him, feeling like "a jerk"? That was exactly how he wanted you to feel.  That is what is called manipulative guilt-tripping in order to try and get you to feel sorry for him and change your mind. It's creepy and not okay.

So, my answer to complete strangers who hit on me in the street is: "Thanks for the offer, but I'm not interested." The end. You owe him nothing more.