I’m due in August so I’m thinking more about how i want to feed my baby when he arrives.
It's good to think about these things in advance but be aware that baby may have their own ideas too 
I originally wanted to breastfeed however I have generalised anxiety disorder which is currently quite bad and am worried the lack of sleep will cause it to become unbearable for me. I’ve been having trouble sleeping during pregnancy (hence the post at 6.30 am - I’ve already been awake 3 hours 😖) and it has made my anxiety absolutely crippling already.
There's so many variables that it's really hard to advise - you may find your anxiety dissipates at the birth and you enter motherhood feeling stronger and calmer than you've ever been. You may not. You might have a breastfed baby who's a good sleeper. You might have a formula-fed baby who's a terrible sleeper. You get the idea...
I have been considering combi feeding with breast milk and formula at the same time, but wondered if it’s possible to exclusively pump the breast milk as opposed to feeding from the boob so that it takes some of the pressure off me? Or would my supply dry up? This is all assuming I am able to breast feed in the first place.
Different women respond differently to pumping - some can breastfeed just fine but don't get much out with a pump, others express fine. While expressing does take some pressure off - my DH can take baby while I get 5 -6 hours - it comes with pressures of its own. Personally, I don't think there is one perfect way to feed a baby, and it's about weighing up the pros and cons for each individual baby and their mum to work out what that is. And it may change over time. Would your supply dry up? Dunno - there's a lot of different factors that impact on your supply. There are things you can do to help it though.
I really don’t know what’s best
No one does, we're all winging it.
I mostly want to ensure the baby gets the colostrum when he’s first born as I’ve heard that it is good for their immune system so don’t know whether to try breastfeed and see how long I can manage, but I don’t want to feel like a failure if it doesn’t go well.
You might feel bad if you don't give it a go - you may be someone who finds it pretty straightforward. You don't know until you try. If you try and then stop, you'll be in good company. 80% of British women try breastfeeding. By six weeks old, 24% of babies are exclusively breastfed. By six months it's 1%. (UNICEF stats, if anyone cares). 34% of six month olds are combi fed though, so clearly a fair few women find it works for them.
Obviously you'll have heard the "breast is best" slogan. In terms of the nutritional factor of the milk that's true, but there's more to caring for a baby than just milk - mother's mental health, for example, is very important too. I think you really do just have to wait and see and then make the decision that is best for you and your baby as a whole.
I’m a first time mum so really not got a clue, are there any classes I could look at online or anything that could help? Any experiences you’d like to share?
Lots of people recommend La Leche League. They definitely know their stuff, but I sometimes find their tone veers into quasi- religious, and prefer kellymom (website).
Ask your mw what support is available - where I live there is FA apart from a peer support group that I couldn't get to for the first 6 weeks due to a rough CS recovery, then lockdown started. My friends in the next NHS Trust over had unlimited access to a multi- disciplinary award- winning breastfeeding clinic, so you never know.