This is genuinely tough, and I'm sorry you're dealing with it. The short answer is that it depends heavily on your state - inheritance laws vary a lot. Most states have what's called "pretermission statutes" that protect children who are accidentally left out of a will, but the key word is "accidentally." If your father intentionally excluded you, that's usually ironclad legally. However, some states presume that if a child isn't mentioned at all, it might have been an oversight rather than deliberate disinheritance, especially if the will is older or doesn't explicitly say why you're excluded. You'd need to look up your specific state's rules or, realistically, talk to a probate attorney who knows your state's laws inside and out.
Beyond that, there are a few other angles worth exploring with a lawyer. Some states allow you to contest the will if you can show it was made under duress, that your father lacked mental capacity, or that there was fraud involved. There's also the question of whether your father had any outstanding debts or obligations toward you - though that's a different legal claim than inheritance rights. If your relationship had genuinely improved before he passed and there's evidence of that, it might matter for certain legal arguments, though again it's state-dependent.
The reality is that you need professional help here, not just internet advice. Many probate attorneys offer free initial consultations, so call around in your area. Bring the will, any documents about your father's estate, and be ready to explain the timeline of your relationship and whether you think the exclusion was intentional. Acting quickly matters too - there are usually time limits for contesting a will.