Before you get a first aid kit, get the training in first aid, CPR and Stop the bleed classes. These can save your life, or someone else's life. Since you want to put together your own kit, it should be somewhat designed for your level of training. (I.e if you do not know how to suture, why carry a suture kit.) You should be able to take care of several things.First - Carry anything that you need for your own medical issues. If you need any prescription medications, an epi pen, diabetic gear etc. Make sure that you have those items. Second- cover all the little things that you may see day to day. So carry some different sized band-aids, some small gauze pads, some medical tape, etc. Also carry some basic over the counter medications for headache, anti - inflammatories, anti diarrheal and stool softeners, as well as anti histamines. You also want to carry some sort of tablets like Tums or Rollaids. These items are the basic items for a good personal first aid kit. If you want to go bigger, then start to carry items to combat those things that will kill you. Have a way to provide an airway and keep yourself safe in doing so. Carry items to stop major bleeding. Items such as Quik-Clot, a Tourniquet, and trauma dressings. You should also carry a way to seal a chest wound (If you know how to do so).
These are the basics that you could carry. The most important item that you can have is training and practice. The items in your kit are useless without some type of training in how to properly use them.
If you would like, send me a message and we can chat and I would be happy to help you set up a kit that is designed for you and your level of training.
In case you are wondering, I am a Firefighter/Paramedic with 30+ years in the business. So I think I may have some insight into what makes a good first aid kit.