With my constant mood changes why do I continue to make life altering decisions?

It sounds like you're feeling a lot of confusion, you recognize that what you are doing isn't working, yet you can't seem to find a way to make the choices that you want to make. This is a place many people find themselves in and feel like it is impossible to get out of, but the important thing to remember here is to take things in baby steps. 

There are a number of questions that need to be covered here. First of all, you have a question about a potential diagnosis of bipolar which seems to potentially underly much of your behavior. For that, you would need to meet with a provider for a diagnostic interview or psychiatric intake, typically done by a psychiatrist or psychologist.  There does tend to be genetic predispositions with certain types of mental health disorders; however, just because it runs in your family doesn't mean you have bipolar disorder.  Sometimes, growing up in a situation where certain behaviors are the norm leads to others adopting those behaviors as their own. Once the individuals realize it and get out of the situation, they can make changes that are more within their personal norms and feel more comfortable. 

When we find ourselves avoiding tasks that we know we need to do there are a few different ways to handle the situation. First of all, working on your mindset and realizing that saying yes to one thing means saying no to another. As you sit down or head out to do something, ask yourself first, "If I do this, what am I saying no to?" just making a habit of stopping and thinking about the choices that you are making, while it may not fix everything immediately, is a step in the right direction. It is important to remember that you have been living in this pattern for a long time, so even making the decision to change is the first step, but the steps to follow must be made in small manageable chunks where you can easily see success so your brain will see the reward and then want to continue making those changes.  

Create a system that only asks for small changes of yourself, instead of focusing on the large changes that you are wanting to make overall. If you want to start a regular exercise routine, think about when is the time that you will be best motivated to do something, and instead of making a large goal like: I will go to the gym and workout for an hour at this time; start off with something that is just activity that you like it may be just committing to 15 minutes walking in the neighborhood, or 15 minutes dancing in the living room. Once that 15 minutes becomes a habit then increase it and gradually it will become something you look forward to and you won't have to fight yourself to get it to happen. 

For budgeting, creating space between you and spending you money so you have to think about it helps with starting to get in the habit of saving.  An example may be setting up a savings account that you don't have easy access to from your checking account. This ensures that savings are only used for emergencies or for the purpose the money was set aside for. 

With relationships, if you find yourself in an on-again-off-again situation; it's important to sit down and ask yourself why that is the pattern you are following. Are you comfortable and happy when you are alone? If not, then you will find yourself compromising your own happiness and goals in relationships, and they will be unpredictable and potentially border on toxic for both of you.  It's important to be happy and content with who you are before you add in the complications of a relationship with another person. 

You have a lot to unpack here and if you try to focus on everything at once, it will be overwhelming. It's important to pick one or two areas of focus and take small steps in those areas, make those small things a habit before you try to take on anything else.