How do I change?

So many starts and so many times I have given up before really even starting. I will go to the gym for a day and not go back for a week, two weeks, or a month. I have to stop this cycle. I have to stop making excuses. I want to be stronger, physically, mentally and emotionally. For my boyfriend, for his boys, for my daughter and more importantly for myself. I just turned 31 and was so far down in the dumps I did not know what to do. I have been getting bad news left and right and I know that all of it is my fault because I have let myself go. I haven't been caring about myself. I keep saying that it is time for a change and I really do believe it. I just don't believe in myself to do it. How do I change?