It might be worth going to the GP for yourself. You are very anxious and rightly so as a ftm but I do worry your anxiety might be a precursor to PND. Please do talk to someone. nobody is at all perfect and you sound to be doing a wonderful job.
Gaviscon makes them constipated. We were given it for ds's reflux at 5mo, but the constipation was more distressing for him than the reflux! We decided to just use it with his bedtime feed or if he was distressed, and wipe the puke up the rest of the time! If it is being used for reflux in your case and you want to continue its worth going back to the gp and discussing the constipation, there is other medicine they can give that doesn't cause it, but they try gaviscon first
Same as CurlsLDN DD was on it for reflux but the constipation left her in more pain and distress, it did get worse and in the end she didn't go for 2 weeks and when she finally did it was rock hard marbles. She was EBF so not normal. I stopped the Gaviscon completely but she still suffered from constipation until she was about 2.
I do understand how bewildering being a first time mum is. How desperately you want to do it right and how difficult that is if you don't have a close friend or family member nearby who has done it before. It's not that long ago I was in that position myself.
But I've seen quite a few really worried threads by you and I think you are being very hard on yourself. Speaking to someone in real life might help more than we ever can.
It may just still be the chaos of hormones at five weeks. Or the shell shock of parenthood - don't underestimate that. But PHD can sneak up on you too. Do get checked out and look after yourself. People on here are always happy to help, don't ever worry about bothering people. It is just that there is only so much 'computer people ' can do.