Over the last 3 years I have been having a tough time. I nearly lost my unborn twins as they suffered from twin to twin transfusion .After they where born 9 weeks early my partner left me soon after. This was a stressful time for me as I already had a daughter that was 7 at the time. All I want to know is life always going to be tough for me or am I ever going to find happiness.

Life is tough for everyone, and happiness is something you create yourself.
Sorry to be so blunt, but that's the truth.

I think happiness starts when you stop relying on others to make you happy

Well, as a semi-pro palm reader here in the Great State of Texas, I bin reading palms for nearly 20 years and have read thousand of folks. You don't state your age but I can tell you that when folks turn 30, life gets much more manageable for both mens and wimmens as the hormonal levels drop off a bit and the brain AKA the Big Head gets more say as to how we react and what we do. OUr culture really encourages folks to be Drama Queens so unlearn that ASAP as it does little for you but entertains others at your expense. It really isn't about you. Best advised is from Sylvia Brown" 'Be Kind and help each other. Don't B*tch . Go Home (Heaven) when your time is up here.' It's just another lifetime in a long string of many - God is Eternal and since He/She made you out of His/Herself (First there was only the void and then God said, "Let there be Light' and so on), then you are part of God and Eternal as well. Most Christians don't read nor understand the basics of their own faith so I hope you are better versed than many.
A good read is 'Conversations With God', a channeled work but more great answers as to why we are here and what we are 'supposed' to be doing here.

When an egg. A carrot and a coffe bean are subjected to the same harsh environment of boiling water, the react differently. The egg hardens up. The carrot softens and the coffee bean changes the water around itself Deecide what u want to be in these rough times