"I am not smart enough"-why do parents and teachers feel the need to compare?

Sometimes i feel lonely and hopeless about my future life because i have failed my GCSEs and could not do A levels.

I got: C(maths, English, German) B(Religious studies) BTEC level 2 science, Business studies and Health and Social Care(pass x2)

It took me two shots to get my English and maths, but at least i passed them.

I now currently do the level 3 Health and Social care BTEC and still wondering where this will take me.

I don't see myself in any career because i feel like i am not smart enough for anything.

My big sister is 18 and opting for medicine and so is my little sister. I just feel stupid when i compare myself to them especially my younger sister who like 10x smarter than me and undergoing through GCSEs-she did like 4 times better in her mocks than me when i was in year 11.

My dad will sometimes compare our abilities academically and this makes me feel like a piece of **** because he does not seem to expect anything great of me due to the fact that i have ADD and a learning difficulties.

One of my friends in sixthfrom already got 5 offers from her choosen unis, whilst i am happy for her, i am disapointed in myself for not working hard enough at GCSE to be able to do A levels and be in her position now.

I don't know what to do, i feel lost and hopeless, even thought of commuting suicide.

I just want a better future for myself and be able to live comfortable and provide for my parents when they are older.

I need help, anyone?

Any advice.