It's good that you are planning to visit your local sexual health clinic. Because of the tests they will do, you will find all the answers you need. Your mention of syphilis indicates that you are starting to panic. Quite frankly, I don't think you need to be concerned.
The more you look at your penis, the more you see: "little red spots", "white stuff", "a little bit of discharge". The "white stuff" is probably smegma or maybe dried semen and you can get rid of it by retracting your foreskin in the shower and washing with a mild soap or just water and the friction of your fingers. The red spots and your red foreskin are probably just the result of the normal action you give your penis
The two swollen lymph nodes in your neck could be from a viral infection -- pretty common during winter. But that's just a guess. You need facts, which is what a doctor can provide.
Have you thought about talking to the diabetic you had sex with? Perhaps that person needs to be contacted.
I repeat: I don't think you need to be concerned. Stay calm and try to control your fears. I know that's easy for me to say -- but I understand what you are going through. All the best, mate. You will be ok.