the 707 was probabaly very very old and this leads to huge maintenance and fuel bills, this is what is crippling you now, you are best off declaring bankrupcy if this aircraft isnt bringing in any money.
start fresh and start small, turboprops/ small jets to local airports within 200/300 nm away should get you started, make sure the aircraft are not too old (i try to keep my fleet less than 12 year old). also check the condition of the aircraft, it should be over 70 percent to help stop technical problems affecting delays and cancellations.
keep an eye on your company image, this is an important aspect of the game, always have a marketing campaign in place and do your best not to let this drop below zero.
once these aircraft have started to make a profit you can either keep going with the same sized aircraft or think about something bigger like a Boeing 737/ Airbus A320. These planes will bring in a lot more profit but with added expenditure so make sure the routes you place them on have enough demand. try to have as many flights a day out of all your fleet, (all mine fly from between 5 and 6 in the morning and dont stop until midnight).
this is just a basic overview of what i personally would do, each airport requires a different strategy, others might be able to go into greater depth