Text strings are just that: Strings.
So I assume you are talking about text in some browser object, e.g., inside a <div> or <span>? Note that you can *NOT* change the style of text that is in the value= of any form field.
So, yea, you have to use the various HTML controls. Which means <b> and <i> and, almost surely, <span>.
<div>Now <span style="color: red;">is the time</span> for
all good men <span style="font-size: x-small;">to come</span>
<span style="font-size: xx-large; color: maroon; font-weight: bold;">TO!</span>
So I don't know what you mean by "it is a stubborn string", but the best thing to do would be to show it to us and tell us what you want it to look like.