1. Assuming that you are now using a TextBox: all changes to any TextBox Font property will set the font-styles, size, etc., for ALL content. If that's what you want, you may want to skip the rest of this comment :)
... edit : removed incorrect content: properties of a Font of a Control, like a TextBox, like 'Bold are read-only ...
a. looking at your specific code: why are you bothering to reset 'bold and 'underlined booleans to false ?
b. you already have good answers here to show you how to combine style flags using logical-Or, so that's taken care of
c. if you want to define FontStyles in advance and re-use them: it's pretty easy:
FontStyle fs1 = new FontStyle();
fs1 = FontStyle.Bold;
textBox1.Font = new Font(textBox1.Font, fs1);
FontStyle fs2 = new FontStyle();
fs2 = FontStyle.Underline;
textBox1.Font = new Font(textBox1.Font, fs1 | fs2);
2. But, assuming that you want to make a text-editor where some of the Text can be underlined, or bolded, but other parts of the Text can be plain, or in different font-sizes, or even colors:
You need to change to using a RichTextBox.
Here on CP are several excellent examples of extended RichTextBox based editors: suggest you search for them, and study them.