Like bloodshot said, HOLY SHIT!!!
Not cool, you really really really, seriously shouldn't ever use more than 80% of a breakers capacity. Right off the bat a 1000w light has about a 10 amp spike at startup, so use that and maybe a fan on one circuit and get that space heater on a seperate one asap.
DO NOT RUN YOUR OP ALL ON ONE CIRCUIT!!!!
most breakers in homes only handle 15 amps before they trip, if they're cheaper breakers they might not trip, which sounds awesome, until your house goes up in flames. Seriously, address this issue right fucking now!!!!!
If you have to have your high power devices on an extension, it is a good thing you got the 12 gauge, but it will not solve your load issue.
For the love of all things good and holy, please fix this immediately.
Not trying to freak you out, tripping breakers may not seem that serious, but i'm telling you it is, i've been to ops where the breakers were taking twice the safe load and not tripping, let me assure you, you would have to be "tripping" to let this go on any longer.
Good luck!!!