YANBU
I had a 31 weeker who was in NICU for a month. He was exclusively BF and I was pumping at home and trying to establish BF whilst he was IV fed in hospital. He also almost died of pneumonia/meningitis at a coupe of months old.
Thinking of your DC corrected age they must only be about 3.5 months.
Absolutely fine for you to say you are not going. It is unlikely you will enjoy the experience as you will be fixated on how your DC is doing.
I would calmly and clearly explain to your DP and the gift giver (who was a bit naive in buying tickets for someone potentially with a young baby even if you had not had a preemie.)
If this was a wedding invitation you would have had everyone stating how it's an invitation not a summons. This is a gift. If it causes you heartache and upset it is not a gift at all it is not a nice thing to do.
You are doing a cracking job and coping really well. Unless you've had an exclusively BF preemie baby other people won't understand.
You might have to talk about your DC as age adjusted as when you say 7.5 months people think of a partly weaned baby not a couple of months old who might not have started weaning yet as he was so early.
You are absolutely doing the best for your baby by putting them first and exclusively BF them for as long as you want. Nobody has the right to tell you you should leave your precious baby in the care of someone else. Trying to keep BF going whilst you are away you will have to express in order to keep supply going and stop you feeling
uncomfortable. In a festival setting you will have to pump and dump as I can't see how you could effectively store your milk safely.
As someone who struggled to establish BF due to an emergency CS I realised how precious that milk is and was collecting tiny tiny amounts in a syringe. It would upset me to have to pump and dump.
For the sake of your mental health you have to make a decision that is right for you and your baby then consider your DP. The gift giver would be ay down my list of considerations.
You are doing great don't let anyone tell you otherwise. Walk a mile in my shoes springs to mind.
Good luck with whatever you decide.