Hugs to you OP. I ended up exclusively pumping for my twins who couldn’t latch properly- pretty sure it was tongue tie looking back but we had a lot more pressing issues to deal with - I was pumping anyway as they were tube fed so I kept going.
Firstly, it sounds like you’re using the wrong size flange (funnel bit) if repeatedly slipping off. Over time this will cause trauma - in the meantime if you have a stretchy bra you can spare, cut a cross in each cup as a makeshift hands free pumping bra.
Consumer grade pumps are often a bit crap and often don’t come with different size flanges. I had an Ardo Calypso double pump which was brilliant, not expensive and comes with different sizes. Argos sell them and often have same day delivery on some things or amazon will deliver by Tuesday
www.amazon.co.uk/Ardo-Calypso-Double-Plus-Breastpump/dp/B00JNCTAKW/ref=mp_s_a_1_1?dchild=1&s=baby&keywords=ardo+calypso&sprefix=ardo+cal&tag=mumsnetforu03-21&qid=1608498828&sr=1-1
I think Ardo do next day delivery.
www.ardobreastpumps.co.uk/shop/products/buy-a-breastpump/pump-up-the-volume/
If you can’t get on with the one you have, I would get another. Good thing about Ardo Calypso is they’re a closed system so you can sell them on when you’re done.
Google the Stanford pumping technique - theres a very dated but very useful video. Combines pumping with breast compression to maximise supply.
If you want to try latching again then google the flipple technique - a way of getting a deeper latch often suggested when there’s a tie. Try some other holds - one of mine could only latch at all if lying on his right hand side, so experiment. Also have a look at rugby ball hold and laid back Feeding position, and try when he starting to get wriggly rather than crying with hunger - you can google hunger cues to show you what the earliest signs are.
Unfortunately lots of tongue tie services are limited during COVID to those cases where the baby is not gaining weight etc. Definitely ask and be assertive but also have a look online to see if anyone can do it privately at your home. I wish I had paid for this in hindsight but I didn’t.
I pumped every 2 hours round the clock at first which is hard going but unfortunately important to keep supply up.
I remember only too well how desperate I felt when mine wouldn’t latch - try not to panic, keep your supply up as best you can, try to get the tongue tie sorted, if you can afford it get some advice from an IBCLC lactation consultant. Just because he can’t latch now doesn’t mean it’s over - I know plenty who’ve pumped initially then managed to establish BFing, hell I even know people who have relactated after stopping completely.
Hang in there, stay as calm as you can, stanford pumping, frequent pumping, different flanges / pump, deep latch, different holds and feed when calm before he gets too hangry.
You can do it :)