I now see the "change maps" button at the bottom (thanks SheenaTiger). I had been playing 12 planets, 2 AI enemies, on Hard.
this is one reason for a forum

still you have not the full program in terms of hard sins-game as far as it goes with AI
some tips for more hard SP-games:
- no locked teams
- ignore the diplomacy (not hard to do that one)
- if you use pirates on your maps, do not put bounties on AI
- use more then one Star and at least 20-30 planets (more recomended)
- definitly more AI's (at least 4-5)
reasons:
the AI tends to clump together against the human player if the teams are open for change and you are unable to manage (or simply ignore it) to get at least ceasefires with the AI. then the travel between stars takes much more time, so your fleet(s) will be occupied with traveling
regarding the AI-problem (to easy)
i think the developers did not realised how many people might prefer to play SP and maybe other reasons
i for myself managed to end maybe 2 or 3 games till now... well, i do play maximumsized randommaps, tend to start over again (for example thanks to the mars-effect-mod) and am playing other games between long sins-sessions (cause bigmaps with many AI tend to be long, and the AI tends to be very annoying if there are a couple of them ... meaning more then 1 or 2)