How can I train myself to feel good all the time?

You'll get better and better at selectively sifting... Here's the thing that we most want you to hear so that it doesn't feel so hard or so futile to you -- each time you deliberately choose a good feeling thought for no other reason than you want a good feeling thought, it activates a vibration within you that makes it that much easier for you to choose the next good feeling thought. Until eventually what you *mostly have access to* are the good feeling thoughts. You can have heaven on earth right here in this physical body and you can within a very short period of time train yourself, orient yourself to the best feeling aspects of everything.
Abraham-Hicks -- Atlanta workshop