It's all gettign clearer (hopefully your picture will too!)
So, let's clarify:
1) The bedroom TV was connected to the aerial by plugging in a lead to the living room socket, lead running up side of house.
2) The picture was fine in both rooms before
3) You have added a Y-splitter to the aerial lead in the loft, with a lead running down to your bedroom into an aerial socket.
Is the picture now bad at BOTH TVs, or just at the bedroom TV?
If it is just the bedroom TV, then we can rule out the cable from the aerial to the splitter and the cable from the splitter to the living room. In which case it is worth checking your connections to the bedroom.
Instead of a Y-splitter, something like
this is better at splitting aerials.
If the signal in the living room is also bad now, when it wasn't before, check ALL your connections are fine (as in the connectors are properly fitted to the cable.
This article shows how to fit aerial connectors (in fact it seems to detail your whole project
)