In short: Some friends can be jealous, unforgiving and not worth the guilt. If you truly feel like you have tried your best efforts with these friends in the past, please don't beat yourself up! Stay positive that you're a good person and put that effort into practice by trying to make new ones through social clubs of any kind. Do this separately from your boyfriend. Make sure to actually put in the effort if you really want it..and include some deep self reflection about your efforts with others
But the long answer: If the opportunity arises to go to your friends birthday or to invite them to yours, I would definitely do so. It could smooth things over and sometimes people believe in actions over words. To see you putting in effort into hanging out could start off a good thing.
On the other hand, these friends don't seem understanding at all. Missing three events should never mean you aren't friends anymore! That's not very loyal and you do deserve friends that stick around. But I do have a feeling that because they see you spend time with your boyfriend and not bother to see them much that they have given up trying. This doesn't mean you're selfish at all, if anything you're living life and you should be able to decide what you dedicate your time to. But....
This bothers you. So you need to know, people can decide to drop others if they feel like they aren't getting anything in return. So while it's not your fault they are being difficult, you haven't really said what efforts you have gone to with your friends besides wanting to talk to them after they are upset. Sure, you missed three events due to good reasons, but did you ever try to organise something to replace the hang out times you missed?? If not, they feel like you flaked.
At the end of the day, drama doesn't end at a certain age, trust me. And if friends don't stick around for you, maybe it would be good to try to make new friends! Join a club, a study group, sports, or anything you like at your university. I really want you to be honest to yourself and make sure you dedicate time to others and not just your boyfriend. A lot of what you're saying reminds me of when I was 19 and barely had friends. Some were toxic and I'm very glad they left my life! But I realised I missed a lot of opportunities with some friends due to my lack of effort.... It took self reflection to realise I didn't put enough effort into making new friends either. Yet, I put so much effort into a boyfriend. You should definitely try to make friends because boyfriends aren't always forever and you should always try to have your own life :)
You seem like an extremely self aware person who doesn't want to hurt anyone's feelings. I'm really sure if you try hard enough you can make new friends if these ones give you too much stress.