In no particular order:
1) Despite what you may be told by medical professionals and what you might read, you (as a person with adult-onset Type 1 diabetes) are not “rare” or some “minority.” Adults represent the vast majority of new-onset Type 1 diabetes; it is a widespread myth and falsehood that Type 1 diabetes is a childhood disease.
2) Learn all you can, but at a pace that does not overwhelm you. Good sources of learning are Think Like a Pancreas by Gary Scheiner (although he does perpetuate some myths about adult-onset Type 1 diabetes, but the rest is excellent) and my new favorite Bright Spots and Landmines: The Diabetes Guide I Wish Someone Had Handed Me by Adam Brown of DiaTribe. Sugar Surfing by Stephen Ponder and Kevin McMahon is diabetes management gold. Using Insulin and Pumping Insulin by John Walsh are older but excellent, and the Type 1 University (https://type1university.com/) is a great source for learning specific skills. Be wary of the Internet—there is lots of good, but there is lots of very bad.
3) Get a correct diagnosis: many if not most people with adult-onset Type 1 diabetes are misdiagnosed as having Type 2 diabetes. It is important to get a correct diagnosis to get the correct treatment (exogenous insulin); being treated as if you have Type 2 diabetes may be extremely harmful. Get the full suite of antibody testing (Glutamic Acid Decarboxylase Autoantibodies (GADA), Islet Cell Cytoplasmic Autoantibodies (ICA), Insulinoma-Associated-2 Autoantibodies (IA-2A), Insulin Autoantibodies (IAA), and zinc transporter 8 autoantibodies (ZnT8). Don’t just get GADA, because a small but significant percentage of people are GADA-negative but positive for one of the other autoantibodies. Autoantibody testing is the gold standard test for Type 1 autoimmune diabetes: if you are antibody positive, you have Type 1 autoimmune diabetes. The suite of autoantibody testing, full price, costs about $471. The c-peptide test, which shows how much insulin you are producing (virtually all children and adults with new-onset Type 1 diabetes are still producing some endogenous insulin), is useful, but does not provide a definitive diagnosis.
4) Begin intensive insulin therapy as soon as you are able. The correct treatment for Type 1 diabetes, at whatever age it is diagnosed, is exogenous insulin as early as possible, to control glucose levels, slow the destruction of residual beta cells, reduce the possibility of diabetic complications, and prevent death from diabetic ketoacidosis (DKA). Many adults can prolong the “honeymoon” period (the time when some remnant beta cells are still producing insulin) with intensive insulin therapy (including using an insulin pump). Bruce Buckingham MD at Stanford has had tremendous success in prolonging the honeymoon by starting the newly diagnosed on insulin pumps immediately after diagnosis. If a pump seems like too much or insurance will not cover one, MDI (multiple daily injections) is good. Early insulin use and prolonging the honeymoon period will make it easier to control your diabetes and greatly reduce the risk of diabetic complications, thus making your life better.
5) Allow yourself time and space to grieve. The diagnosis of Type 1 diabetes is devastating for most people. As an adult, you may wonder what you did wrong to precipitate Type 1 diabetes (the answer is nothing, it is an autoimmune disease). Grieving often takes a lot of time, more time than we care to admit or allow, so it is important to give feelings of anger, denial, and depression their due. Get the support you need from family, friends, online (folks at TuDiabetes.org (who truly understand), some excellent Facebook groups), and a therapist (preferably one with knowledge of chronic illness) if you want. Do things that bring you joy and well-being: exercise, yoga, meditation, gardening, petting dogs/cats, music, etc. Remember you have been given a second chance at life; make it count. Balancing Diabetes: Conversations About Finding Happiness and Living Well by Kerri Sparling has great tips for living better. An older book that I think is excellent is Psyching Out Diabetes: A Positive Approach to Your Negative Emotions (Rubin, Biermann, and Toohey. 1997). The Behavioral Diabetes Institute (http://behavioraldiabetesinstitute.org/) is also superb in this area.
6) Consider the wise advice of people at the Behavioral Diabetes Institute: maintain the best blood sugar control you can, avoid lows (hypoglycemia) especially severe lows, and live your life. Don’t think that a cure for Type 1 diabetes is coming anytime soon. If you are a Type A personality, be especially wary of being harsh on yourself for some number on a meter and be wary of trying to achieve some “perfect” A1c.
7) Test, test, test. Lots of blood glucose testing means better control. Don’t allow embarrassment to prevent you from taking proper care of yourself (meaning, if you need to test or inject in public, do it.) Eat to your meter (use your blood glucose meter to test your blood sugar after meals and eliminate from your diet the foods that spike your blood sugar). Follow the rule of small numbers (from Dr. Bernstein: big inputs make big mistakes; small inputs make small mistakes—in other words, lower carb means lower doses of insulin means smaller “mistakes”).
8) Get organized; get your security blanket in order. For me, security comes in the form of backup—I carry my diabetes kit with me at all times. My diabetes kit includes insulin vial and needles, meter and test strips, note from doctor, glucose tabs, and backup supplies for my insulin pump. I wear a Medic Alert (www.medicalert.org) bracelet that says I have Type 1 diabetes, an insulin pump, and that I have autoimmune hypothyroidism (Hashimoto’s disease).
9) Use your healthcare team. Find good people who you can work with and who work with you as an individual. Be your own best advocate in the healthcare system.
10) Get tested for Hashimoto’s Disease and celiac disease, two autoimmune diseases that are commonly seen in people with Type 1 autoimmune diabetes.
 Jerry Palmer MD in Type 1 Diabetes in Adults: Principles and Practice (Informa Healthcare, 2008), page 27.
 IAA test does not distinguish between autoantibodies that target the endogenous insulin and antibodies produced against exogenous insulin. Therefore, this test is not valid for someone who has already been treated with injections of insulin. For example, someone who was thought to have Type 2 diabetes and who was treated with insulin injections cannot then have this test done to determine if they have Type 1 diabetes.