I have only ever been in one proper relationship which has recently ended (you can read the complexities surrounding it all in my other posts on this forum). Nevertheless, I do know what a loving and considerate relationship looks like - and it is something to be treasured.
It seems like the timing of everything may not be right. Obviously if your partner is studying, and you are working full time, the ability to do things together is limited. But I think the key takeaway should be - quality over quantity (certainly for the time being).
Your partner is, as you have said, your best friend. This is something that is truly hard to come by. But it is important to look out for yourself as an individual as well.
You have said that you have been doing full-time work for quite a while now, and that in itself can be exhausting, particularly when everyone seems to be balancing their time so well.
It may be time for a holiday, and even dedicate some time away with your partner. But also consider taking time out for yourself. Time away physically, can help clear the mind and help you reassess your current situation.
You asked about 'how to re-train your brain'. This may be something to do with your 'inner critic'. Perhaps with all the changes going on around you, you may be subconsciously comparing yourself to others and feel like you are not measuring up.
But you need to challenge that, because you are doing so well. For someone so young, you have done an apprenticeship and are working full time! So many people your age would love to be in that position. And the same goes for your relationship - to be in caring relationship is something wonderful.
It just seems like it is your anxiety getting the better of you, and you only need to try and counter those negative thoughts with positive ones. If you feel yourself saying 'I am not good enough', know that that is your anxiety talking and counter it with factual statements like 'No, I AM good enough because I have worked hard, done an apprenticeship and earned my job'.
I am dealing with similar self confidence issues as well, and do recommend speaking to a counselor or psychologist if you feel like you cannot sort through all these thoughts by yourself.
I am sending you the best of vibes.