The problem with the string breaking depends on where the breaks are? Several things can cause that (in addition to using too weak a thread).

You can apply a very small drop of glue to the knot. That prevents it from coming untied. However, the glue makes the thread brittle and you risk it breaking near the knot.

By wire, I suppose you are talking about something like Tigertail. With Tigertail (and similar items) you use a crimp bead to finish it off. You need a good set of flush cut pliers to trim the tag end off close to the crimp.

Later,

Hello again,

Here are a couple jewelry supply companies that I have used. Norm Jones