Try to relax. Easier said than done I know but if you relax and it works great. If you don't manage for a particular feed, don't worry use the NG tube and try again next time. With my DS who was 29+3 that worked better. He then sometimes did 2 BF, and then none for a while, trying one on one off was not always the best way as my son didn't want to play.
Do try the rugby ball hold. little ones find it much easier, particualrly if you can hold the weight of your breat with your other hand while your LO is feeding.
Do give yourself time 35 weeks is still a bit small and a bit little to be latching well. Most prems don't get sucking until 34 weeks after all (although my DS had his own ideas about that too). Keep expressing in the meantime. Oh and get you DP/DH to do the tube feeds so you can get some sleep whenever possible. Its been a stressful time.
Don't compare yourself to mothers with term babies. Or even babies who were born at 35 weks and the same gestational age. That is not the baby you have, and comparisons drive you mad.
Do lots of kanagaroo care, skin to skin etc. If you partner is involved get him to do so too.
Try and make sure your LO is awake when you feed. Chaneg them before feeeding but only to keep them alert.
try to avoid the rigid hospital schedule if your Lo is awake and hungry when a feed is not otherwise due. My DS dietician told me DS should snack if he wanted as his tummy was so small he could do what he wanted.
Although I second RL support be careful that its with someone who knows about prems. They are different and a lot of the advice I found was not suitable for a baby who came home at 4lb 14. Has your hospital got a lactation consultant, or a prem support group? What about a SALT, my DS salt was the best for advise on prem baby feeding, and really supportive. they can be much more helpful than the NCT, LLL etc as they are more likely to understand the issues you face.
Persevere, try and stay patient, try not to worry, try to believe it will all come good in the end, take it one feed at a time, and then when it starts to click one day at a time. i told myself I would feed DS until he came home, then until his due date, then until 3 months, then until 6 months, then until I went back to wrok, then until he was 1 and now, well i'll feed him until we stop but I have no idea how long that will be. However accept if it doesn't its not that big a deal (so many prem mums beat themselves up about when feeding fails, which it can do more easily with a prem than a term baby - I am still feeding DS at nearly 15 months, but I know so many people who weren't so lucky. Now I can't believe how big a deal it was then. There have been far bigger hurdles with DS and in the end feeding is just one tiny part of parenting)