Hi guys! I am so new to this forum I still am in diapers!

I am an old fart bicycle lover who needs a new bike and haven't ridden in three years. I don't know what to do about my lardy 300 lb butt but do have a nice LARGE seat and handlebars that rise up a bit, on my $15 Goodwill bike I got about three years back. It is a road bike, narrow tires but two years out in the weather and it's toast.

Now what? I plan to ride the Jordan River Parkway and bike friendly streets of downtown Salt Lake city. How do I know what bike to buy? I am 5'4" and short legged and am afraid a WalMart bike will kill me or else I'll kill/break it. I plan to keep the wonderful "elephant" seat and upright handlebars from my trashed bike, but I am afraid of banging my you-know-what on the stem of any new bike when dismounting. Any advice?
I'm afraid if I go into a bike show I will come out with a $300 bike I'll never like riding and my hubby will kill me.