First off, let me address the "just decide to be confident" stuff. It's a whole heaping pile of BS. I know the people who say this are trying to be helpful, and I can see the underlying truth of it, but after that it goes terribly wrong. Here's the thing: if I could just decide to be confident and have a high self esteem, I would! Do you honestly think I sit around thinking, "I wish I could just hate this and that about my personality; Gee if only I thought everyone was better and more worthy than me; It would be great if I couldn't see any of my talents or gifts; I'd love to miss opportunities for advancements and promotions because I don't have the confidence to go for them."? Of course I don't. Now, I can decide to try things that could help to eventually raise my confidence. Like I could apply for that position even though I don't think I'll get because I don't feel qualified enough. Or I could try that hobby or activity that I have always wanted to do because learning something new usually makes me feel better, at least for a while. But I can't just wake up, click my fingers, and be confident. It's not because I'm lazy or have no ambition. It's because that's not how I'm made. It's like wanting to wake up one day having lost 20 pounds because you decided to. You might make baby steps, and wake up a pound lighter one week, two the next, etc. But if you do manage to wake up one day massively lighter - say from some sort of unhealthy cleanse or something - it's probably not going to last.
Next, there's the "you're using your condition as an excuse" thing. Well, I think you're using your heart condition or severe asthma as an excuse to not run a marathon. Yes... it's like that. Think of self esteem and confidence as the marathon of mental health. For many people, it takes hard work, long hours, ups and downs, failures and successes. Even after all of that, some may never run a marathon. Maybe they'll make it to a 10K, and that's totally ok. They made progress! I have a condition, which affects the hormones which regulate my moods, my happiness levels, and overall the way my brain works. I'm predisposed to have brain events that are confusing and frightening. Every day can be a struggle to feel "normal". If it's not obvious, this is not a confidence builder. When you can't have confidence in your brain functioning as you'd expect it to, it's tough to have confidence in yourself overall. One day I may become as confident as possible, given my predisposition not to be, but I may never have super high confidence and my condition is not a "crutch" for lacking it. And just because Mary Smith with a similar condition is more confident, that means nothing other than perhaps a little hope that it's possible. People with heart conditions and asthma are each affected in different ways. Some may run marathons, others may never be able to. (Please note here, I'm certainly not picking on anyone with either of these conditions - I have neither, and have still never run a marathon!).
Trying to shame me into being more confident doesn't work either. I already feel bad enough about myself. Don't make me feel unconfident about being unconfident. Telling someone with confidence and self esteem issues that you really want to be around people who are more confident (or anything to this effect) only makes it worse. We know that "confidence is sexy", but there's such a thing as trying to hard and putting too much pressure on ourselves. The most likely way to feel bad about oneself is to constantly try to be someone you're not. So while it's great to do things to try to improve your self esteem, play-acting as a super confident person doesn't address the underlying issue, which may very well be a medical one.
If you've never dealt with low self esteem and confidence, you may not understand how serious and damaging they can be. If you were born with confidence and self esteem, I'm truly happy for you. If you weren't but worked very hard and have gained it, I'm even happier for you, and incredibly proud. If you're not there yet, you're not alone. Don't listen to people who make it seem like it should be easy or natural - it may be for them, but it isn't for you, and there many others out there who are feeling the same way. Keep plugging along, know that it's ok to have set backs, and most importantly, be you.