i'm not a hair professional, but i have had extensions exactly as you described so just wanted to let you know you're not alone. it's horrible isn't it?
not just wasting money but feeling like you have been taken for a complete and utter fool
i have had racoon hair extensions when i was in high school (so absolute years ago!) which cost around £400 and then i had extensions done by the girls over in the hairdressing department when i was at college doing beauty and they cost me over £200 and then i had them done a few months ago by a hair extension specialist and they cost me just under £200.
every time i have been left so disapointed - the racoon ones they left a huge chunk of hair without extensions, the college ones were super thin (of course she was practising so they'd never be perfect) the hair extension specialist extensions though sound exactly how yours do! they were tangling literally the day after i had them done and i too had bits of cotton in them. green and white cotton i'm sure haha. i put off telling the extensionist because i just kept thinking they just need time to settle and adjus but they never got any better in the next week-2weeks in the end i had to get back in touch with her. i was told it was because i washed them 3 times in one week, then i was told it was because i had used an anti dandruff shampoo (they were tangling and rank feeling before i washed it though so dunno how that works *shrug*) then it was probally because of a manufacturing fault and then was told the supplier had disapeared or something along them lines :\
if they are that bad then you too should get back in touch with who did them and see what she says. the extension fitter i used was nice enough to offer to fit the hair of my choice (i had to pay for the hair obviously) for free as i wasn't happy (though when you have a disaster the first time it's really hard to believe somebody can do a good job the 2nd time kwim?). but in the end she cancelled the appointment and refunded me in full (even though she was adamant i wasn't getting a refund when i went back to her) so in the end everything worked out well for me - i did spend like a week in agony when it came to sleeping and didn't go out of the house because i felt so embarrassed at the state of my hair haha but nevermind.
i actually posted on here for advice like you because they were that bad, i got the same replies as what you have had (though i later deleted the post after reading the replies because i didn't think it fair that i should put off potential clients as not all extension fitters are bad i'm sure?).
i've been using this site for a little while now and hair extension posts seem to be so common, there are loads of posts just like mine and yours sadly
x