Crooky,
Why can't you do more than 2 lenghts f/c. Do your arms fall off? Do you get cramp? Do you run out of breath?
f/c is 90% arms (or near 100% for some of us) whereas breaststoke is 60%/40%. So your problem may be tiny weeny feeble arms.
But my guess is you end up breathless. In which case practice helps a lot. You are probably not unfit if you can see 112mile bike and 26 mile run as doable. Therefore heart and lungs are probably OK. So it is purely technique.
Do the slowest front crawl you can. Focus on exhaling underwater and getting a nice clean inhale on rotation of the body/head. Stay in control, relax, do not worry about speed. Do not over kick. Relax, relax, relax.
The panic will go, the breathing routines will come, and you will get used to it.
M..eface