7 month old not gaining weight and constipated

Hi guys, I'm in tears as I write this... So any input on the matter is greatly appreciated.

I a FTM to a beautiful 7 month old baby girl, who unfortunately I was not able to nurse since birth. I was absolutely sure that I wanted to breastfeed, so we went to a lactation consultant and found she had a posterior tongue tie. This was around 4 months after birth, and the whole time I was pumping to increase my milk supply.

She had the tongue release and immediately took the breast. I felt relieved since I was sure I had plenty of milk and that feeding would not be a problem anymore.

Unfortunately, I returned to work just a week after her procedure, she stayed at home with her dad and I had to start pumping again during the work hours. Normally, I don't have a problem with this, except I found my milk supply dropped even though I've been "breastfeeding" her at night and whenever I'm home during the day.

Nursing her during the day is extremely complicated since she fidgets around like crazy and only drinks for a couple of minutes at a time. I thought this was more of an annoyance rather than a problem since she was gaining weight normally and since she receives some formula during the day from a bottle.

However, now that we've introduced solids... she's quite constipated and I can tell she's in pain and now I'm really worried and stressed that it is because she's not getting enough breast milk when I "nurse" her. I've been keeping an eye on her weight... And seen things like a gain of 20g this past week.

I'm devastated, she doesn't seem underweight... She has chubby arms and legs... Yet somehow she's gaining 20g in a week, and has gained 400g in a month and a half.

Has she been sleeping through her hunger this whole time??? Was I actually starving my baby this whole time???

I stayed home a lot more this past month and I was feeding her "normally"... Is it this all my fault for wanting to nurse her?

Please, anyone... Have you felt this way or had similar issues?