Has anyone experienced or felt this way before

Don't worry, it will get better.
You can say happy birthday to her, you can wish her well.
It's too easy to fall for someone and give them your soul and energy, it's always a gamble and one day with someone else it will work out how you want it to. But these things can't be rushed, your day will come.
Never forget to take care of yourself first, respecting yourself is the only way to get over something like this, you are good enough on your own.

It normal to feel very nostalgic about previous good times in a relationship. Particularly if it ended sooner than hoped. It eases with time, and new and better love comes along

Thanks I hope so. She was my first love well I thought so anyways I loved her so much and cared about her so much wanting what’s best for her always. Yeah but I’m not sure if it’s reachable to her because she did say to me to just block her and I did at some point 10 months was when we last met 3 months was when we spoke on socials so. Well hopefully your right but in this moment in time gaining my trust is difficult because I believe it’ll only used to be played and wasted as time goes on. Yeah but unfortunately on the mental health side of things it just doesn’t feel the same I feel a lot emptier, sad and upset too and this year has to be the worse year of my life yet because I didn’t see that coming.

Dude, I'm a 43 year old mature student doing GCSE's and I can tell you that life will always surprise you in good and bad ways, don't forget to look for the good ones or you will miss them!
Best advice I can give you is not to look at her socials and force yourself to do something that requires your complete focus when you start thinking about her like study, gaming, whatever it may be because at least it will be some time when your mind is not torturing you! Start with a blast of loud music to give your senses a wake up call.
And to talk to people when you feel empty, or just be with people and don't push them away. This is probably the hardest and most important one I realise.
If it gets bad counselling is a valid option, a couple of sessions to get it off your chest where you will have some support.
I can recommend not waiting too long to do it, it does help.
Peace bro, 👊
https://www.nhs.uk/tests-and-treatments/counselling/

To be honest @Zarek the relationship was something I hoped that would never end. I waited for her, for such a long time because I wanted to look long term beyond past a relationship for example marriage and getting married. I know it’s been 10 months but it’s more of a matter of time than I guess really to heal? Based on this experience I’m unsure about whether there’ll come any better. Because I was so appreciative and grateful of her, loved her for her and wanted her only.

I’ve been in this situation. 10 months is not long to heal. A year minimum and even then it’s possible to still feel wistful years on. In my experience and what I see around me, new and better love does come

It's always devastating when someone you expected a future with doesn't reciprocate in the same way you did. Can you explore the reason behind why you feel guilt for not wishing her happy birthday? To me it seems like you feel responsible for her in some ways. It's amazing to hear about how much you care for her, love her and wish her the best.
I'm not going to be able to comfort you through the hypothetical of this situation, if you will find better love or not, I don't know. What I can tell you is experiencing those same strong feelings I once did for someone, you eventually do learn to live without them in your life. Yes it feels terrible. But you will adapt to let them go gradually.
For people such as ourselves, I really recommend to take caution with relationships, because it seems like you fall hard and give it your all...if anything I think we will benefit from putting that same energy in other aspects of our lives. I do hope things work out for you.

I think that the past 3 years we were able to celebrate it she did say she wanted to remain friends because that’s what she had seen me as yet she still felt as though I was one to her that she cared about most, loved most and considered important but only as a friend whilst I felt more than that for her which I told her. Yeah throughout the relationship I always said to her your my utmost priority, I want to prioritise you and that your always happy and felt loved and always having my endless unconditional affection with care and support whilst wishing what’s best for her and wanting what’s best for her.
I guess I am struggling in the adaption phase. This year has been the worst year of my life. And I do feel quite sad and upset. When you say to put it in other aspects of life what do you mean please?

As repetitive as this advice might come across, you might want to build on those things that are not only romantic comfort that you can fall back on when things get hard, like football for example that is community based. Although I do think having people close to us for support is essential.
I can see how much you value romantic relationship and love, however is there any other area of your life that you want to build on? I would say as an example I really want a stable mental health and focusing on that etc.

I’m really not sure? I saw two people who used to go to the same school as I did and they were literal high school couple throughout and have been to this very day and since today and it made me wonder how if they were able to manage and salvage their relationship why can’t I do the same with mine she’s my first love and just wonder why can’t it be the same.

It’s like opening a can of worms I guess, this kind of stuff will make you feel worse the more you experience it, that’s why many people rely on therapy to explore these deep rooted difficult feelings. You are beginning to process it on your own right now and that can mean biases, maybe you being hard on yourself etc.
Have you thought of alternatives that although high school couple last in some cases there are those that don’t? Maybe she is your first love and nothing can change or replace that special bond you had with her and yet there will be space you can create to experience these same feelings for and with someone else. The first time you fall for someone else again, the first time you tell hug them, the first time you experience your firsts with them. I get that everyone emphasises love and makes first love to be this grand thing but with every person there will be a unique yet familiar experience that’s waiting for you. You probably won’t even be able to compare them to each other because maybe you realise each one deserves its own stage.
What I’m saying is don’t shut yourself off to this idea that first love = magical must be lifetime soul mate connection. I still don’t know hypothetically if this was the best you’ll get, regardless I know there will be more connections to be made if you are open to experiencing them. If not then you will have to learn to cope and manage.
I also think this year being a difficult year for you doesn’t help this hopeless state you are feeding into. I think maybe you should look into learned helplessness and see if any of that resonates with you.
Sorry I typed too much, I genuinely wanted to offer my honest opinion but feel free to only listen to what you resonate with but do consider it openly.

No. Don’t be sorry at all. I’m glad you did offer your opinion and always will be considered to me. So what would therapy involved then?
It won’t be as special that’s for sure and not the same. Yeah I’m probably thinking it way too ahead. But she did have a special place in my heart and I love her so much.

I'm not advocating for therapy in the sense that you go and talk then it's magically fixed, it doesn't work for everyone, but more in the sense that you begin to understand yourself and how you can sort through the types of feelings you are experiencing with your questions of 'why can't it be the same' for you as it is with other couples, etc.
It's okay to love someone so much and it's brave even, still at the expense of your own sanity as it's costing you now isn't what anyone would want to see you go through...be kinder to yourself, be your own friend, treat yourself, understand yourself, comfort yourself, because as much as love can be offered from others, it can be offered by yourself too.