I caved and relapsed
How are you feeling about it?
I am really disappointed with myself
Relapses can happen with any kind of addiction so it is okay to feel disappointed that it happen to you. It would help you if  remember the next time how disappointed you felt to stop yourself from relapsing.
I'm feeling down because I quit and I'll never do it again.
Well don't beat your self up too much. Addiction can be a real struggle. You need to pick your self up and keep up the hard work.