I found that the more pressure there was to get them to bed at a certain time for whatever reason made me more likely to get frustrated is is just wasn't happening. Give yourself some tools to help you cope with it - things like going outside for 5 minutes to compose yourself when you feel you are at your absolute wits end. Or on the nights where you really have to get into the uni work get DH to take over for a while. But you are not a horrible Mum for feeling this way OK