Yet another scrub in need of guidance

Hey all,

Title says it all really. I'm a dev and I work in apps and functionality for websites but my experience with SEO has always been limited to the minimum essential. But then I wanted to see if I could earn a little extra and I made my own ads site for.. well, it's adult stuff, I figure I'd mention that because I imagine linking to it for examples would be kind against rules; which is why right off the bat I don't include links to show what I've been doing.

In any case, the site has been created since March, I've done my best trying to optimize it and, in fact, Google's speed insights gives me a positive score on both mobile and desktop versions. I've researched the keywords and for a while I even got close to having the site ranked on the first page (lowest I've ranked was 13) but then came September and I was perpetually tossed into the third page.

So here're a couple of questions that despite much trying and researching I couldn't solve on my own:
1. This is the one that's driving me nuts the most; the page indexed by google on results is not the site's main page. It's just a post within the site. No matter what I've done or tried, the one thing it indexes on results is that one post. I've changed my sitemap to exclude that page, I resubmitted it and even so, it still shows that page on the results as opposed to the main page.
2. I've been keeping an eye on the competition since day one and with the exception of #1, I honest to God can't figure out why the other sites rank above mine considering that their scores on optimization are very poor, their page designs are quite bad and one of them hasn't even been updated in months.

Since I am trying to kick off an ads site, it's not like I can post content to generate traffic. The ads I've so far are for people I sorta know and wanted to give a hand by giving them a freebie (plus, in turn they are helping me) but if I can't rank within the first 10, the chances of generating interest in hiring space on my site are null. Which in turn means I can't generate content to rank better.

Anyway, any kind of insight on what I could be missing or things I could try to improve #2 or fix #1 would be extremely appreciated. I'm learning and part of that is knowing when you need to ask for help instead of repeatedly bashing your head against a wall.