Mistake number one : toying with personal management. It kills company image, which kills your income. There is very few to win by managing personal by hand, and so much to lose. Set it back to automatic, and forget about it. Definitively.
Mistake number two : well, I need more info to know exactly what is your mistake. Though 737-400 have a "too small" limit around 1800NM for international flights(and probably around 2300NM for domestic flights, I don't remember exactly). If you fly them on a 3000NM route that has more than 200 demand, well, passengers just expect a widebody airplane instead. They just won't take your plane. In current GW3, I had a 737-800 route to Khartoum from Orly. That's 2478NM, above the "too small limit" of this fleet group. 270 demand. 60% Load factor. Replaced it by a A330 : now at 85% load factor, and making plenty of money. Other possibilities(less likely) are overpricing the route to insane levels(like 2 times the standard price) or oversupplying the route(which seems not to be the case).
If you land a 150-seater on a 450-demand route, at standard price, your LF should be around 80% from the beginning. If you land 2 of them at the same time, only 40/45; 3 of them, 30% or so. planes covering full demand should be "full" only when Route image is at 100%. More or less(depends on several other factors). What is your total capacity on the route, the total demand, your pricing, the route distance, the route image?
Mistake number three : overextending yourself. There also I need more information to be sure : are those airplanes newly built or from the Used Market? If they are new, mistake number three is just taking new planes while beginning a new company. It never finishes well. If they come from the used market, it's more subtle - the proper thing to do is to order just one, then to set up the lines(and buy the slots), see how much it costs, and check if there is enough cash to take another one.