Being ignored by friends

Does anybody else feel completely ignored by their friends? I have certain friends that never reach out to me first, it will always be me who has to start the conversations. Whilst I do understand that the majority of people I speak to are neurotypical and have busy work lives I feel that if they truly wanted to be my friend they would try and connect? One friend in particular tends to very rarely post on social media or message people so I tend to try sending messages when they've recently posted things as I know they'll be on their phone, but it can go days or even weeks before I'll even get a response. 
It breaks my heart because it takes two minutes to check in with someone and say hi or to let them know you can't talk right now, and this friend is someone I have known for at least 8 years and they were my best friend in high school. Now I have an odd thing with relationships, I know I can be overwhelming and honestly I feel entitled to be responded to but I've worked on that and rarely message them anymore. I don't push people to respond to me but I really really struggle internally when they don't. 
I don't work myself due to extreme social phobia and mutism, so I think it's hard for me to understand their point of view about being busy. Do I give up on the friendship? Do I try and salvage it? I think it's worth noting that I have told them multiple times how I feel about them not responding and they've apologised and said they will try and be better with it but then there is just no change?