Go to the doctors and ask for Lactulose and Fibogel. Eat prunes and oranges, drink prune juice (if you can stand it) and loads and loads and loads of water. For the love of god do not sit on the toilet and push like you can do when you aren't pregnant. It will NOT end well. Sit with your feet up if you can, like on a little stool or something and lean forward with your elbows on your knees, and just breathe. Just trust me on the not pushing front, you will regret it.
seconded for prunes and the stool (you can buy Ines called squatty potties or something but a toddler's toilet stand will do the trick!) I also found hot water with a slice of lemon first thing helped and loads of fibre and water made a difference. This symptom, thankfully, didn't last long for me and by about 14 weeks I felt better.
I feel for you, it's horrible isn't it. I found the best thing for me was LOADS of water, fruit and vegetables, and weetabix to keep things regular. When I couldn't actually go, I got lactulose and all it did was make me bloated and gassed up. I turned to doner kebabs and curry puts road right through me
I take a big spoonful of chia seeds daily with a large glass of water, sorted me right out. I also switched from gaviscon to rennie which has made a big difference... although rennie not as effective for heartburn