Rep Power: 0

Hello everyone
This is my first time posting to these forums. I have a few problems with my car and I have no idea what to do. I am getting really worried about all these things because I really can’t afford to take it to a mechanic. I have a 98 honda civic ex 4 door.
The first problem is that whenever I am driving and I push the accelerator down, it goes, but around 30-40 mph, the rmp starts to drop a bit. So I push it down a bit more, and it takes a few seconds before the rmp jumps up again. This happens quite a bit, especially on the highway. I also noticed that the car doesn’t have the same amount of power anymore
Second, when this started happening, I noticed that whenever I open up my gas cap, a lot of pressure seems to be released. I know it is normal, but I don’t think that this much is normal.
Third, whenever I turn my car off, I hear an air leak from around the firewall on the driver side. I think that it is from the brake booster. How can I find out where the air leak is? My brakes have been bad for so long now. I just replaced the pads and rotors last year, and now they are shaking so bad whenever I brake.
Fourth, my speedometer doesn’t work anymore. This just happened today
My check engine light is on, and I have three codes. I lost two of them, but one of them is p0136: circuit condition (heated oxygen sensor bank 1 sensor 2). Probable causes: oxygen sensor defective, fuel system very rich or lean (check other codes first), engine misfire condition repaire, fuel pressure very high or low-fuel pump or pressure regulator.
The other two codes were about the same as these.
I really don’t know what to do or where to start. I am a broke medical student. I replaced my fuel pump, and o2 sensors. Please help!

 
Rep Power: 812

-for power loss, try changing spark plugs ($2 ngk v-power will do just fine) and plug wires (ngk) and also the distributor cap and rotor.

-air leak you can spray the vacuum lines with a soap/water solution and see where bubbles form when the car is running.

-rotors warp easily, try not to brake over 40mph or ride the brakes more than 5 secs next time

-speedo could be the cluster went bad, or the speed sensor, or the wiring.

-you need a new primary oxygen sensor, be sure it is the oem plug in type and not a universal unit. you may also need a new fuel pressure regulator. have the pressure checked before spending money on parts tho.

 
Rep Power: 235

You can also carefully spray carb cleaner and listen for the idle to surge.