This game is honestly making me miserable at times

I honestly don’t know what to do to get better at this game guys and I’m getting truly sick of it now. Every time I kill someone there’s someone else there to get me. Every time I try and be careful someone appears behind. So often I seem to unload into someone as they come in the door or round the corner and they down me in less than a second with what seems like aimbot despite me having the jump on them. I just don’t get it. I’m not even that bad a gamer in other shooters I’ve always been around middle or towards the top at times. Sometimes people even get me through what looks like solid walls and stuff. I just don’t get it and I’m depressed. I need some comfort and inspiration from my fellow players. Thank you