Replaced battery won't work, stuck in boot loop

Ok, so my iPhone battery has become progressively worse over the last couple of months(or so..) so i thought it would be smart to replace it! but i didn't want to do ship it to an apple store and pay loads of money for them to do it, so i bought it myself. i opened the phone today, unscrewed the philips screw holding the battery connector in place, and the started trying to get the battery out, but it was really stuck in there, so it took a while to get it out, even had to put some guitar picks(thin) in there(but not where there was any cables

though). i finally got it out, so i simply put in the one, connected the cable, and then i tried to turn it on(with wall charger in of course), but it turns on for like 5 seconds, then turns off again(boot loop). i have reconnected it several times, and i tried to boot it without battery, and the same thing happens(boot loop). so i have no idea of whats wrong here, and i really don't think i have damaged anything in the process!

thanks!