The poor lamb Kayz. Sounds like you've tried loads. A couple more though (can you tell I've been there? This is how ds1 started and ds2 struggles a bit too). Yell at me to shut up if you have tried it all.
Get a potty. Sit her on it every day after dinner. The position is good for getting things moving and it can help to establish a routine. She won't go right away of course but keep doing it, gradually it will help as she gets older.
water water water water water. I know I said that last time but it is vital. WAter based drinks and breast milk. If she won't take water but will take squash then go for it. You can wean her off squash more easily than you can get a bowl to recover from the consequences of severe constipation.
If you spot her straining then grab her, nappy off, legs in the air. Wait and cycle the legs. When the poo starts to come gently apply pressure to her bum area to support it. If the poo is big and hard it can hurt. By supporting it you lessen that pain and you help prevent little fissures which can open up and hurt next time she goes. By making sure it doesn't hurt you help to avoid the cycle of witholding because it hurts.
Also, lactulose will soften the poo to make it comfortable to pass but it will not MAKE her go. Which is why stuff like potty after dinner is important. Most people poo after a meal.
Diet. Plenty of fibre, check you are getting enough soluble fibre. ALso dairy is often found to be constipating. You could try cutting back or eliminating completely to see if it helps. If cutting back then try to keep it to cooked dairy so let her have normal family meals with dairy in but don't give her yoghurt for example. If you eliminate completely then dairy takes 4-6 weeks to get out of your system so don't expect instant results. I wouldn't worry about you giving up too.
If she won't poo and you can't get teh bloody lactulose in/it isn't helping then go and beg for some glycerine suppositories. They really will work! But only use them as part of establishing a good poo routine.