I have noticed a pattern where I tend to shy away from confrontation, but as a result I stew over things and then (quite frequently) bitch about them privately with friends afterwards 
So if someone is rude to me in a minor way, instead of calling them on it I'll pretend it's fine but then afterwards I'll moan to friends.
How can I reduce this behaviour? If I politely challenge rudeness or thoughtlessness will I then be able to let it go better?
Case in point - I helped someone earlier today and they were very ungrateful and didn't say thank you. Instead of calling them on it I stewed and then moaned to my friend and ended up bitching about the person in question
which I know isn't a healthy approach.
So will 'being assertive' solve this? Any experiences?