wen i start car in mornin it struggles turnin very slowly then battery goes flat an i hav 2 bump it, its completely flat wen i leave the car standin 4 a few days then try an start it

wen i drive along the lights/speedo are dim, an wen i use heated windscreen the revs drop to zero but car is still driving, wen i turn heated winscreen off the revs shoot up to 1000-1300 rpm then the roblem goes away after a short time of driving, it also cuts out wen im breaking/slowing for a junction the revs drop to 300 then it cuts out,

i went to the garage got multimeter on it read 14 volts, so i bought a new battery an it only lasted 7 days an started givin me exact problems

any ideas cuz my rs is stressin me out dryin up my bank account 24/7.....an im on my wits end an ready 2 sell her but i wont cuz i love it, jus want to get it running sweet.......i miss them days

plz sum 1 help, ive read endless topics jus cant figure it out