Pumping less than I should b ?

So my baby is going to be 12 weeks tomorrow so he’s almost 3 months. he’s like two months four weeks would say. I was breast-feeding for the first month exclusively. Then I started introducing formula more and more but I still want to breast-feed and I’ve been trying to pump to get my milk back up but nothing is coming out like it should I am literally making like 4 ounces within a day. I did palm within the first two months and I was getting like 4 ounces in one session 15 minutes 2 ounces each boob. Now 15 minute sessions, give me like one ounce or half an ounce. Any advice on what I should do or if this happened to anyone before ? Its just hard when I go out or if I have a night out and I drink or something and it just throws me off and I want to get my milk supply back up. I’m trying to pump every four hours which is six times within a 24 hour. Period. But sometimes I skip out on one or two.