Unsure what the issue is.

Try getting your doctor to prescribe one of the ED tablets eg tadalafil. It will most likely solve the problem and will also allow your confidence to build enough to stay hard without the tablet after a few successful sex sessions. 

Just to look at what you say that is really interesting: -

Every time I’m with a girl - it requires a great deal of focus and effort - Masturbation is fine - I wouldn’t say it’s stress or anxiety - I’m not worried about performance or impressing - in saying that I do have really bad health anxiety - It’s a worrying issue for me as it always seems to happen - I’m a healthy ... it shouldn’t be happening.

As Pepasan suggests, if you take the tablet approach you may be able to build your confidence anyway and put the 'performance issues' behind you.  However, I have repeated the words you have used yourself so that you can actually REFLECT upon what it is you are telling yourself!

You believe you have a really bad health anxiety.  This is unlikely to go away - even if you deal with, and overcome the sexual intercourse issues.  I wonder if you have shared your concerns with a medical professional?  Where is this belief coming from?!  Why is it an issue for you?!!  Of course, it could be based on fact(s) that you have built-up in you own mind that are causing you endless concern!  Even how we interact with others - particularly family and friends, can have a real big impact in the way we see ourselves, and lead to us making self-assumptions that are always not based on fact.

So, i.) get your health concerns checked out with the correct medical professionals (if you live in the UK that is relatively easy to do); ii.) I wonder if you are really relaxed and are enjoying yourself when you are masturbating?  For example, do you do this in a relatively rushed manner to get it over and done with?  Whatever you do when you are on your own - you need to start making these something really good: see them as self pleasuring, learn to stay relaxed, learn how to stay focused, stay with the feelings you are experiencing, the entire pleasures.  Once you have built up confidence in the times when you are self-pleasuring you can start to build-in the same practices before and after you have sexual intercourse.

Once you can do this for yourself, think of the person you are having sex with: how can you heighten their pleasures? make it a real experience for them?  You say you enjoy oral, do you give orally too I wonder? - simply, again, to make the whole encounter as relaxed and enjoyable as possible for the BOTH of you!!!

You can make the whole sexual encounter instances better for you, and more satisfying for your partner (at THAT time, if you do not have 1 regular partner in the near future) without attending to your self-confessed "really bad health anxiety"

I WOULD SUGGEST you need to work on both - particularly the health anxiety, as this is a major concern for you, that, is not just going to go away itself - you need to do specific work on that to attend to it, work through the specific health concerns you have, and hopefully, in the not too distant, near future, begin to put those issues behind you.

GOOD LUCK ......