I had the same problem with DD not opening her mouth wide enough, so I really feel for you. Not sure what will be best for you and your LO but here's what I did...
I tried all sorts of things and got advice from BF counsellors etc but eventually got to the end of my tether (and my nipples were v sore) so decided to express for a couple of days and have a think about what to do.
I also struggled to produce enough as the day wore on, and she'd get increasingly frustrated - pummelling my boobs (especially the left one). I felt like I was doing nothing but feeding, and each time it was stressful and agitated (her and me).
During my couple of days off, I used Tommee Tippee Close to Nature bottles to feed DD EBM (and used formula for the dream feed at 11pm, after much soul searching). I wanted the option to go back to BF if I could so thought the boob-shaped bottles might be better than using normal Avent ones.
Amazingly, after a few days off and bottle feeding EBM, I put her back on the boob completely spontaneously because I felt like one side might be getting blocked up and I knew she'd be the best way to clear it. She opened her mouth perfectly and sucked like a dream. I think the TT Close to Nature bottles had actually re-taught her to open her mouth properly in a way that all the other tricks I'd been shown didn't. Also, I didn't bother about carefully positioning her and I just plumped her on a cushion off the sofa on to my lap and away we went. Before, we'd got to the point where it seemed I had to precision position her for a good latch or I'd be sore.
I think just having a break, her remembering how to open her mouth, and me just feeling like I didn't have to BF if I didn't want to made the whole thing less fraught.
We now do a BF every morning and she has 4 further feeds. The other 4 feeds are 2xEBM and 2x formula (200ml each feed). Sometimes one of the 4 bottle feeds is replaced with a BF if I'm out longer than I planned and can't go home to express. If I BF more than twice a day I do get a bit sore, so the latch hasn't been fixed entirely, but it's definitely better than it was.
And the early morning BF there's always loads of milk so I don't ever worry that she's not got enough like I used to.
Sorry for long post. Hope that helps a bit. Good luck, you're doing great. I know the ideal is to exclusively BF til 6 months but I have to say, once I made the decision to mix feed everything fell into place and it's transformed my life. Now if we could only get her sleeping properly...