Beeswax is your friend!
When you handsew, especially if you're using linen thread (which tends to break easily), you need to make your thread as strong as possible. The trick is beeswax. Get a big chunk of it and keep it in your sewing kit (you can usually find it at art supply shops). Before you thread your needle, run your thread through the beeswax a few times. I use my thumb to press it down into the wax as I run it through, but find the way that works best for you! Regular candlewax will unfortunately not work...it's gotta be the real deal!