First of all, kudos to you for your strong work ethic. Your ethics clearly show in your concern about how this will impact your future employment opportunities.
As for what you should do, there are a few things you can consider. It sounds like you are doing the best that you can to leave your current position on a good note. This will help increase your chances of a good reference from your previous employer despite the absences you've had. Sometimes things just don't work out, but if your current employer knows that you did your best to manage the situation, they may be more supportive of your efforts to try again with a new employer.
Now that you're working on an as needed basis, do your best to follow through on any promises you make and maintain open communication with the employer. If you're still not able to meet the employer's needs on an as needed basis, be honest with them and realistic with what you offer. Employers need to know they can count on you, for however much - or little - you promise you can provide.
Next, think about whether you want to be proactive or reactive about the situation. One option is to be upfront about the situation with potential employers. This doesn't necessarily mean disclosing your diagnosis. That is a separate issue for you to decide. But you can consider whether you want to share information about how you experienced some personal medical issues that took some time to get under control. If you decide to be upfront and provide this information, make sure to end on a positive note. If this diagnosis taught you how to be more organized, let the employer know. If it taught you empathy for your clients, share that with the employer. Reassure your employer that you are a good candidate for the position.
(If you think you do want to be proactive with this information, review our disclosure information to make sure you understand your rights in regard to disclosure.
Another option is to say nothing. Since this was your first job, the employer may just consider it "growing pains" in the work environment. If your previous employer is willing to give you a good reference, and if you've done what you can to learn to manage your symptoms so you can be a successful employee moving forward, there may be no need to have a discussion at all.
MS is unpredictable, and there's a good chance you may experience a work hiccup again. Try to use this experience as an opportunity to learn about your rights and responsibilities, and explore supports to help you manage your MS in your personal and professional life.
Best wishes to you!