One space or two after a period? + dialogue punctuation question

I wrote my novel with two spaces between sentences, but I've been hearing more and more that there should only be one. I know that the two spaces is a bit antiquated, but honestly I'm finding it difficult to break the habit my seventh grade keyboarding teacher drilled into me! How important is it? Are the two spaces too distracting to potential agents/publishers? Should I go through and delete spaces?

Also, when using an ellipsis, is it better to put a space between the periods or not (. . . vs …). I’ve seen it done both ways in books, but which is the most correct? And on that note, when using dashes in dialogue is it more correct to connect the words or have it separated by spaces? (ie. Like—this, or like – this).

Thanks!

For your own work, do what you want - most modern word processing and display systems overwrite the manual spacing after periods anyway.

If you are submitting, then go by the submission guidelines of the agent / publisher.

In most word processing applications it is an easy "Find > Replace All" command to change your (period two spaces) to (period one space) or vice versa.

Indeed, most word processors have it built in so that ... with no spaces turns automatically into a properly formated ellipsis. Same with double spacing after a period. They add the space making two spaces seem gaping. again word--this makes a proper dash in Word where word -- this wouldn't. It's all meant to make typing easier rather than old typewriter problems. I know people these days aren't taught to double space after a period (I wasn't and neither was my brother).

But yes, you can get rid of double spaces easily after the fact with replace all.