It is hard being pregnant when running round after other children but it's the way it goes. I'm 22 weeks and have a 20mo DS so I know how you feel.
Does your little one have a nap still during the day? If he does then try have one too at the same time or at least pop your feet up and do nothing while he sleeps.
Early nights are great, pre-pregnancy I was in bed between 12 and 1am, now I'm in bed for around 10-11pm and it really does make a difference.
If you wake in the night for the loo then don't turn any lights on because they'll just wake you up more - pee, wash, and back to bed. I you're uncomfortable then try some pillows in the bed, I find one between my knees eases hip pain or one under my bump eases back/side pain. If heartburn is an issue then sleep on your left side and pop an extra pillow under your head/shoulders - I was swimming in extra pillows by the time I had DS!
Snack on things like bananas, peanut butter (or other seeds and nuts), yoghurt, and oranges - they're all high energy foods and will help give you a little boost. Avoid big meals (I always feel sleepy after a large meal - food coma!) and instead eat little and often - I have breakfast, a mid-morning snack, a small lunch, an afternoon snack, a small dinner/tea, and a small supper snack. Make sure you're drinking enough too, it really does make a difference and I find I'm sluggish if I don't get enough over the day so I carry a bottle of water wherever I go and sip throughout the day.
Put off whatever jobs can wait. Yes, I like to vacuum all of the carpets every day and mop all of the floors but can I be bothered right now? No. So I vacuum the living room and passage as and when they need it, get DH to do the mopping, and do the bedrooms once a week. I can go back to my normal regime when the baby arrives (or not!). Get your DP to help out wherever possible, even just little things like doing the dishes while you chill will help.
If anyone offers to take your toddler or pick up some shopping for you or whatever then snatch their hand off! My brother took DS to the park in Saturday and it was fab having a few hours to myself. My mum made mince and dumplings last night and knew DH was working late tonight so put up a plate each for DS and I for me to heat up for our tea tonight (DH will fend for himself when he gets home). DH phones on his way home from work each day and asks if I need anything bringing in to save me a trip to the shops.
Make little changes where you can but don't beat yourself up about being tired - we're all allowed to feel knackered when we need to! And the plus side is that no one is pregnant forever, you're over halfway through now so it's just a matter of waiting it out.