Welcome JohnB47! I'm afraid you are face to face with a couple of the shortcomings of the "Story Editor". It is easier and more productive to bypass the story editor entirely than to fight it, and as far as I can tell, you lose no functionality. If I am wrong there, I hope a more sophisticated user of Scribus than I will step in to correct me.
I should state clearly that I love Scribus, the "Story Editor" notwithstanding.
My first suggestion is to have the "Properties" dialog open all the time. The "Properties" dialog allows you to directly interact with your written matter without the intermediary "Story Editor". Ignore any suggestions in the Scribus Wiki that the "Story Editor" is the way to fly. It is not.
So, first off, you can add text directly to a text frame - just copy and paste or write directly for short work. Once you have created a text frame, double click within to place the insertion point (and the attention of the program) inside the frame. Then you can simply type or copy and paste text within.
Should you wish to modify the properties of only a portion of the text within, select that bit of text and then use the "Text" tab within the "Properties" dialog. You can change point size /height of selected text, or change the font, or boldness / weight, or if the typeface has them, change to italics.
You can change line spacing by very fine increments - select "Fixed Linespacing" and fiddle with the points spacing between lines. I'm using a different version of Scribus so if my description varies from what you see, it is almost certainly that they keep improving Scribus and that what I see is different. Regardless, line spacing appears to be applied to paragraphs and not to individual lines. The trick here is that you can make Scribus see a single line as a paragraph by hitting "Enter" at the end of a line. Don't do this unless you need to because it can make other things more fussy. But if you want to fine tune a single line break it off by treating it as a separate paragraph.
In all these bits of tuning, you will immediately see that changes you are making. Much easier than the remote control of the "Story Editor".
I should speak to how important it is to put the attention of the program where you want it. If you click outside the frame you wish to modify, nothing will happen, and you will sometimes be misled into thinking something is amiss.
If your frame is active, it will be outlined in red. If you have single clicked, the frame border will be red and show tiny circles in each of the corners and in the midpoints of each edge. Now the program is attending to the frame itself, and any change you make will change everything in the frame. Use the "Properties" dialog to adjust your text. If you change, say, text size, all text in the frame will change uniformly to your new setting, even if you previously had created various sizes.
If you double click within a text frame with text in it, the focus of the program is now different. The focus is off the frame and on the text itself. You should see an insertion point blinking within your text so that you could type in more right there. A second visual confirmation that you are in this second mode of focus is that the red frame border has lost the little circles, and now the frame has just straight red outlines. Now that your focus no longer upon the frame itself, but is within the frame, and you can now drag to select portions of text and use the "Properties" dialog to fine tune properties like size, boldness, spacing, and only do so to your selected text. Again, the immediate response of the actual text means you can see your results immediately and that makes it far easier to fine tune what you are doing.
I'm sure I have written something poorly so please ask any questions that I have left unanswered. Or anything new that occurs to you.