You said "we use some affiliate links in the newsletter but they don't really bring in any revenue"
If your affiliate links aren't bringing in revenue what makes you think and ad will bring revenue to and advertiser?

You said you have a list of 2500 targeted people, what is the open rate for the newsletter and the click through rate, how many click on the links.

By Newsletter what do you mean? Is it an old school newsletter filled with a bunch of stuff. If so that could be part of the problem with the affiliates. Newsletters can be overwhelming and too much stuff for our information overload society.

Try sending one topic per email at one per week. 80% value & 20% promotion, meaning either 8 out of 10 emails provide value and only two promote something your's or someone else's, or 80% of each email is value and only 20% promotes something.

If your subscribers on YT are active & you videos have lots of views to the end, that may be a better place to look sponsors. Tim Schmoyer of Video Creators has some videos on how to do that. Also I would use the channel to continue to build the email list.

Guard you list and only present them things you are willing to back up 100%, if you guard them they will become your raving fans and will buy & promote for you. Continually put things in front of them to buy especially from other people and they will grow weary and move on.

Schedule a time if you'd like help deciphering your analytics or reviewing your content.

In addition to Jason's excellent advice under promise, over deliver. When reporting on KPI's make sure to show any additional KPI's they are getting that may not be obvious, their benefits to the partner and any extra services you've thrown in above and beyond the agreement.

I agree with Jason, but for a different reason.

One if you do web development, you'll still have to market yourself.

Two as a consultant, if I'm going to refer someone to a developer, I want one who has an understanding of marketing too, because a web site isn't just about being pretty, it also needs to convert into leads.

Regarding traffic, it depends on where your target market its, what resources you have already with your personal accounts, and what you're most likely to implement on a consistent basis with your limited time and resources.

In terms of "sign up to your site" I'm guessing you mean sign-up for your email list. Again this depends on your audience, but it can be something simple, like a video or pdf as long as it provides value and your message touches your target on an emotional level. This may take some testing a tweaking and you won't know until you get enough eyes on your opt-in to see if it's working.

You said your offering a service that is free, but it's also a business. Money has to come into play somewhere. To get people talking, help them with their problem. Send me a message and I can send you a link to a service that runs $1 a day that may be of help.

Without knowing what you've sent or done it's hard to know what to advise. If your efforts are all about trying to get in front of someone to say look at my stuff and the and the features it has that's an up hill battle and won't be very effective. There's ways to warm up the leads with social media if they are using it. Look for ways to provide value and build relationships with the decision makers and the gate keepers. A sales coach might be able to help with ways of getting past the gate keeper.

Studying hard indicates a good work ethic which will help with your entrepreneurial efforts.

Do a search on dyslexic millionaires and you will see people like Richard Branson, Steve Jobs, and Steven Spielberg on that list. My guess is they also had low GPAs.

Grace & Lace which appeared on Shark Tank & Tom's Shoes are just two examples of companies making a profit and donating.

People are first going to be attracted to the solution your product or services solves. The charity component would be a feature that would help them to decide for yours vs your competitor.

Before you "rack the shot gun", you need to work on this "I'm actually a little nervous about the response being overwhelming" If you don't reframe that subconsciously you won't give you marketing efforts 100% and it will hold you back.

Also, you client isn't the medical clinic, you have two clients the decision maker for the clinics who approve the purchase and the users of the app who will help convince the decision maker of the benefit. For the decision makers, I'm thinking trade journals off and online, If employees would be using the app they or the decision makers gate keeper may be on LinkedIn or other industry specific platforms.

Put keywords related to the news you're sharing in front of, or instead of "the bulletin" Also share it on other platforms like LinkedIn, Pinterest... ask people you know to watch & like it, especially in the first 24hrs after you post it.

Everyone has offered great advise. I see two things mentioned by David & Huebert, needing elevation. Providing value to the 800 you already have. Are they opening and clicking?
You can add people to the list, but if they aren't responding what good is it. If you're providing value all your other efforts will be much more fruitful.