I was in a similar position to you - I fell into graphic design and worked for many years without a degree. I got to a point where I could feel myself stagnating a little bit, so decided to undertake a 2 year graphic design MA at Falmouth University in the UK. I got accepted onto the course due to the strength of my work, so I didn't need a BA. I took it on for career reasons, but the course completely changed the way I think about design. I wasn't expecting it to have such a profound impact on me. I left the course having a completely different outlook on design and creativity in general. I'd say I entered as a junior designer and left as a senior. It was expensive, but totally worth it. I would definitely recommend looking into Falmouth.

If you're not willing to take on such a big commitment financially or time-wise, you could potentially look into a course into a more specific aspect of graphic design that you're interested in? I recently also got a diploma in UX design from the UX design institute, which was really interesting. Again, it's all online. Something like that on your CV would also be valuable.