Don't be afraid to talk about what's going on with you. What you're going through isn't terribly unique, so I'm sure a ton of people here can relate to what you're describing. Personally, I went through quite a bit of what you're talking about myself, both at school and at home. In fact, many people have grown up feeling like you do, so you're not alone. Hopefully that fact alone makes you feel a little better.
First of all, more people than not grow up feeling either picked upon or like outcasts in high school. Sure, knowing that probably doesn't really help in that moment when the kids are talking shit about you, but but I really strongly urge you to remember is that it won't ALWAYS be like that. Remember, everything changes! In just a couple years, you'll be out of high school and (I hope for you!) off to college.
In terms of blowing off steam, there have been a few really good suggestions here. Exercise (whatever is suitable for your heart condition) is an especially good one. Not only does it keep you physically fit, it also releases endorphins which help keep you happy.
What I'd caution you AWAY from are drugs and alcohol. Both are widely considered by counselors, therapists and the general populace unhealthy means of dealing with anxiety/depression, can form life-long addictions and, especially for someone your age, are also illegal. Besides, from what you describe, you could well be battling elements of depression, and alcohol is a clinical depressant. Why on earth would you want to do something which will only make you feel worse?
My suggestion would be to find extra-curricular activities that you enjoy, especially academically, and pursue those. Staying after school for an athletic event, art class, science project, to work on the school newspaper/yearbook, etc., can not only pay off when it's time to apply for college, it can also keep you out of the bickering at home.
Consider volunteering as well. I know it sounds hokey, but doing something positive and constructive for your community -- whether that's mentoring younger kids, acting as a tutor for learning challenged peers, etc. -- can work wonders to improve self-esteem. Someone once told me, "The easiest way to build self esteem is through esteemable acts." It's true! Volunteering is a great way to feel better about yourself (and in turn forget the B.S. others have to say about you).
Besides, you never know, it might also help you meet and make new friends.
