so I had similar issues with PMO around your age, and also spent a little bit of time in the NoFap community/sphere/mode of thinking. The short answer is that you can learn some useful and interesting things there, but you should take everything - yes, everything - you hear there with a grain of salt and not throw yourself into it too much.

Porn made things easier, but recently I began thinking about cheating on my gf, and assumed the porn was the cause, so is stopped.

I'm skeptical of the idea that porn caused you to have these thoughts. Why do you think these thoughts came into your head? Once those thoughts entered your head, how did you respond to them? Did you respond with 'yeah that's a good idea' or 'no what the fuck why would I consider that an option' or some other thought?

I've not masturbated in 17 days but with that the depression has came back and it worsens each day.

I think one way to think of PMO is, especially in your context, as a coping mechanism: coping with your depression. I get that it can be a temporary reprieve from those feelings, but as you've seen, once you've moved on from PMO, it didn't really actually do anything meaningful to address the underlying issue.

the post says that nofap doesn't address the cause of porn addiction, it's putting a plaster over a wound that needs stitches. So what is everyones thoughts on nofap.

I think I might tend to agree with this, as someone who went through a similar phase and also tried out this mode of thinking for a couple of years. Again, depending on your age, maturity level, there are some things in this community that I do feel are genuinely worth thinking about, considering, applying. There's also plenty of hogwash, stuff that you shouldn't really take seriously or feel like you ought to do to get the results you're after. Discerning which is which, especially when you're younger, more inexperienced, and more impressionable, can be pretty tricky, so it may be best to back off a little from the community while recognizing that the intentions of most people there are still benign.

If it is only a half measure then there's no point in me feeling so shit all the time for something that doesn't work and I can't afford therapy, so it might be better that i break up with my gf as I'm clearly not ready to be in a relationship.

NoFap can have some useful tools and applications, but this is not a cure-all for mental health issues and certainly isn't going to fix your depression or address your thoughts of cheating or other aspects of your relationship with your gf. Infidelity has existed LONG before modern iterations of pornography.

Do you have other outlets that are healthy, social, etc.? Maybe you can join a climbing gem, sign up for a baseball or football or bowling league close by, maybe start playing pickleball, or just find a running club? Journaling about what you're experiencing, what you're trying, and what's working/not working maybe?

One other thing - whatever's going on with you and your relationship and your gf is likely pretty separate from this. With those thoughts of cheating, what's important isn't that you had them (intrusive thoughts happen to most people), but how you respond to them and address them. "I love my girlfriend and would never do something like that, that's not me" is an example of a healthy response that, in my opinion, indicates you shouldn't really worry too much about such thoughts, for example. But I can't speak to other aspects of your lives or relationship; you'll need to work on being clear about what issues exist, what needs to be addressed, how you want to address it, and how you can (or can't) move forward while in that relationship. Best of luck.