Best way to biuld confidence is to fly as much as possible in as varied conditions as possible.
you cant avoid a thermal as you cant see it, but you can do a bit to avoid hitting the worst of them, if you fly in the morning in the summer you almost certainly going to be flying in conditions that will worsen.
If you fly on a summers evening, nine times out of ten conditions will be improving.
dont worry about hitting thermals all they will do is make you go up, wich is kinda what we want to do aint it?
to be honest getting caught out in there air and thrown about a bit can be a good thing, it gets you used to what can happen and lets you know you are not going to sudenly fall out the sky.
its all part of the learning curve and your flying skills will be richer for going through it.
oh and another thing, a lot of times people look for cumalus clouds or verticel building clouds as a sign of thermals.
clouds will only form when there is moisture present, you can still have strong thermals and no cloud, paragliding pilots call them "blue" thermals.
Enjoy