Okay, so this is a brilliant start! Well done for getting this far, it's certainly not easy by any means.
From what you've said, it's likely experience that is holding you back. When I was Director at an agency we generally wouldn't consider an individual for an established position if they had no experience outside of education. Your online portfolio is an excellent start but you need real terms experience, so it's a good idea to look at entry level positions in the field so while it may not be the job you want right now, it will help you get where you want to go. I'd recommend looking at internships or assistant roles at games/production companies, or use a site like The Dots or Creative Jobs to see if there are any short term/temp roles that you could apply for to bulk up your experience.
I don't know what you mean by C++, but it sounds like a requirement that you don't have. Essentially nobody will hire a person who is lacking a key skill for a role when they have no experience but if you gain some experience and work on projects in your own time, you'll be able to demonstrate that you're capable of completing the project well despite not having that skill which gives you a far better chance of being considered.
There's a misconception that generally when an employer is asking for 2/3 years experience (for example) that they mean specifically in that role, when quite often (in the Creative industry at least) it means 2/3 years of experience in that part of the field. Say I had someone apply for an established position with me, they had a great portfolio with clear ability but no experience, their application would more than likely be an instant 'no'. Compare that with an application from someone who has worked a few years as an intern/studio manager/runner/assistant, with a moderately put together portfolio, they would more than likely be who was offered the role. I know it sounds unfair, but speaking from experience I want someone on my team who I know I don't have to babysit. If you've worked in the industry for a few years, I can trust that you have a general idea of how to deliver assets/liaise with clients and deliver in an appropriate way. If I own an agency/brand etc and I have an established client base, I will not risk that for anyone therefore I won't be hiring anyone who has no/minimal practical experience in a professional environment at at certain level. I know it's disheartening and it certainly is a very difficult field to break into let alone the very niche field you mentioned but please don't let it get you down or give up on what your long term goals are. You will receive more 'no' than 'yes' when you apply for jobs, it's just part of it but keep improving and keep working on yourself not just professionally but as a person too.
Does your CV come across as appropriate for the Creative Industry? You said it's professional but you haven't said which profession, it's important to tailor your CV to the field you want to work in - receiving a very formal, all text, overly verbose CV is an indicator of a lack of experience in the field. Have a look at some examples of creative CVs, The Dots used to be fairly helpful in that respect but I haven't checked it in some time so I can't say for certain now. It's clear from your post that you're incredibly frustrated and trust me, that will read in your applications.
Take a step back to work on yourself and your skills, not just related to the field. If you can't find anything immediate in terms of entry level positions in that sub-field then have a look at other areas where you could work and transfer over the skills. Apply for production runner or studio assistant roles so you can develop yourself in other ways like working in a team outside of your peer group and adapting to on the fly changes in plan. It's great that you have a clear idea of what you want to do, don't lose sight of that but you need to work towards it as more of a long term goal. There will be tonnes of people applying for the same job whenever a posting gets put up and more than likely they could all do the job, but employers look for what makes you different from everyone else, would they be the kind of person we'd want to have on our team or would we be hand holding them until they got the jist. I'm not saying that applies to you specifically, this is broadly speaking. Take a breather and start with something more achievable and to be quite honest you'll develop a far better network of contacts starting with a role like I mentioned above which could take you anywhere. Being able to do the task isn't enough when evaluating whether someone is right for the role, what can you bring to the table for a company that's already established?
Familiarise yourself with the entire process end to end - create an entire project from scratch with a time limit. Put together a full deck as if you were delivering assets to a client within a certain time frame, 10 days for example so include pitch, moodboard, 1 key concept and then 2 alternatives, revisions, several versions, final product, delivery before deadline and post project evaluation i.e strengths/weaknesses. Don't half arse it because it's not real, this will help you find your weaknesses. Use this project as an example in your portfolio to demonstrate that you are capable of continuing to deliver assets to a high standard within a timeframe given.
This is all generally speaking because I can only go on what you've said, but certainly you need more experience overall. If you need any help with CV/portfolio/cover letter then give me a shout and I will do my best to guide you on what to work on etc. Like I said, don't give up because you've sent loads of applications for the job you want and haven't got it. You haven't got the skills you need for your goal yet, so work on building that and find something that you can apply yourself to now.
Best of luck!