Parting ways and moving on is a common and healthy aspie activity. I've had to drop lots of friends in my life, due to anxiety, toxic behavior, them being a general negative influence in my life. Normally I would suggest slowly backing away, making the interactions further apart and making up some health excuse that causes them to back off. Next time I need to do this I'm thinking of pretending I have nits. This all saves retaliation on their part and can end as well as possible.
However. In your case, with the gambling and such an age difference, I would advise abrupt. Assuming there is no damage she can cause to your life in retaliation, I would say break it off quickly before you relapse. Therefore, explain to her face if possible (if she is unstable then use the phone), and explain that you want to move away from toxic pursuits and build a life for yourself. Say things like 'you care a great deal for her' or other such mushy crap, and then make it clear that you won't be in touch again. Hopefully she won't throw things or threaten suicide (I've been there, not pleasant, but people react the way they react and you have to accept this). So I would say get on with it. Also join a support group if the gambling resurfaces, that's not healthy, especially with our obsessive and addictive tendencies.