Manager wants me higher in possession but I'm worried about the space behind me

Your confidence to go should come from the structure behind and around you, not simply from whether the space looks inviting. Before moving high, check three things: has your team secured possession, is there cover behind your side, and where is the opposition's immediate threat if the ball turns over? If the centre-back has shifted across, the deeper midfielder is connected and the ball carrier is under control, your higher position may actually help the team by pinning their winger back. If none of that protection exists, going at the same moment can make the team very open. It may help to stop thinking of the space behind you as 'your space' all the time. In possession, responsibilities can rotate. The important question is whether the team has protected it, not whether you personally are still standing there.