1. We don't need to sustain this many people 2. We don't need to sustain this standard of living
The first one is pretty straight forward, although it's going to take another 100 years to sort out (barring a major disaster). Shifting child bearing years from the early 20's to the early 30's and changing cultural habits so that each family has only 1 or 2 children will do this for us. However to fix this problem, people need to be willing to share the wealth of the world a little bit more equitably. Improving the situation of women around the world has to be a very high priority as well. But if you look at what's happened in the last 50 years or so, it's pretty impressive. I actually have a lot of confidence that we will reach a slightly declining population within the next century. Either way, I just don't see a problem. Fixing this issue will make the world better, not worse. Obviously there are difficult economic challenges to consider, but again I think if we solve them it will only make life better for everyone.
Standard of living is similar. I think we're probably going to have to cut back a certain amount. But I don't think this is bad in any way. We're used to doing things like driving anywhere we want to go in a personal car. But if you go to Europe these days you will find that many cities and towns have opted to do away with cars in their town centres. This reduces some convenience, but massively improves the centre of the town. It means you have to park on the outskirts and either walk or take public transit into the centre.
Likewise we can get rid of a lot of things we don't need. I would literally ban clothes driers if I were king of the world. What a useless waste of energy. You are very slightly inconvenienced by having to hang your clothes outside. Of course you can't do that immediately because many apartment buildings have no access to the outside. Again, over 100 years we can dramatically improve this situation.
Other things are the way we heat/cool/light buildings. Why should it be 21 degrees C every single day inside regardless of the weather outside? What the heck is wrong with having seasons? What's wrong with having a night time? Some people would obviously rather have it the way it is now, but given that I personally live with an apartment that goes above 30C in the summer and below 5C in the winter, I can attest that it doesn't dramatically affect my standard of living. In many ways I prefer it. I was in Tokyo a few weeks ago. There are many more people on the streets in Ueno at 2am than at lunch time. Yes, it's fun, but is this really essential to our culture? Do we really suffer as a society by having to sleep during the night?
Most of our energy usage is in manufacturing and transporting things. We may have to do with less. But imagine if goods were 2 or 3 times the price they are now. Suddenly there is a reason to make higher quality goods with craftsmanship. If energy is priced higher than labour, suddenly there is a reason to train people to do a craft/skill. Suddenly there is a reason to buy a frying pan and not throw the damn thing out until well after you are dead. Seriously, do we need to buy crappy furniture, cooking utensils, clothes, etc, etc and replace them every 2 or 3 years??? Is this really a higher standard of living?
I remember reading a small book on the ecology of older cultures. You would buy 1 or 2 nice sets of clothes. When they got worn, you would wear them for every day use. When they got too worn for that, you would cut them up and make cleaning rags out of them. When they got too worn/dirty for that, you burned them for heat. Is this really worse that having a closet full of clothes from 30 years ago, waiting for a revival of that style? Good grief, clothes end up in land fill and we manufacture brand new rags for cleaning which also end up in land fill. It's almost criminal.
Enough rant :-) Like I said, from my previous rants on the subject, I realise it is not such a popular viewpoint, but I invite you to consider that dealing with constraints does not necessarily make your life worse. We're spoiled to a certain degree and having to do without has the potential to let us grow as a culture.