Combination feeding... - help please!

I am a first time mum - my baby is 2 weeks old and breastfeeding started off great - she latched well and I fed on demand.

2 weeks down the line she shakes her head a lot when trying to latch, to the point she ends up screaming because she's not being fed because she's not latching.

Also now when she is feeding she's spending up to 2 hours a time feeding! She will then go 2 - 3 hours without a feed then spend another 2 hours feeding. It's soooo draining!

Because of this I haven't had much chance to express, but the other night after her 2 hour feed she guzzled down 3 ounces of expressed milk! Then still breastfeed for a further 20 minutes.

Last night I had no choice but to give her formula milk because she wouldn't stop screaming and would not latch, and after this she was sick a few times and now o feel terrible.

She was born 7,7 and is now back up to her birth weight.

I really don't want to give up breastfeeding, it makes me feel like such a failure! I have been considering topping up her feeds with formula but I don't know if this is a good idea or how to go about it.....

Wonder if anyone could give me some advice!..... x