Humane Immunodeficiency Virus, abbreviated as HIV is a virus that once, you are not seropositive anymore, attacks the dendritic cells, macrophages and T cells. The latter are important for your immune system, and you will be heavily compromised if you have none of these anymore. You are extremely vulnerable to diseases and may die of common viruses and bacteria if you are heavily immunocompromised. Medication will and may keep the virus in check, but the virus may mutate so certain medication won't be effective anymore after a time period. Seropositivity is where the virus is latent (sleeping in the cells of your body) and does not cause harm, you may not notice any symptoms but still may be able to infect partners through unprotected sex, sharing needles, or breastfeeding. Seropositivity may last several years, even longer, and then the virus will manifest itself. Depending on your lifestyle and battery of medicines you may live quite long, there may however be some side effects from the medicine you take in, such as nausea, anemia (abnormality in red blood cells), diarrhea, dizziness, fatigue, headaches, vomiting, pain and nerve problems and rash. Medicines may be cheaper or even free depending on where you live and on the healthcare you receive.
There is a three month time window after infection before the presence of the virus can be tested. Even if your latest test within that time window says you're negative, but have had unprotected sex with someone you could still be infected.
It is not a dead sentence indeed, and it may take some dedication to live more healthily and take all your medicines on time, and you may have to suffer a few side effects. The WHO has recommended people to take PrEP, which supposedly reduces the risk of contracting HIV by 92% if you have unprotected sex with a HIV infected person, this combined with good protection during sex, and the person is taking his medicines faithfully there is hardly any risk to engage in sexual contact. PrEP is already assigned to people if they are raped, and they have to use the medicine within 48 hours after sexual abuse and every day for a month. People who fear they have come in contact with bodily fluids of HIV infected people may also use the medicine, such as healthcare workers who interact with HIV patients. There are side effects to the medicine as well, since they're a diluted version of the regular HIV medicines (I'm not quite sure about this).
So use protection when you have sex, use new needles, be careful with breastmilk (but I suppose we're no longer the age we need that anymore) and if you work in healthcare be careful of blood of patients. Get regularly tested if you have sex with strangers or with multiple persons, and if your healthcare system covers it (and if you really want to), take PrEP.