My suggestion is to start by making a promise to yourself not to volunteer your services (unless your services are needed in an absolute emergency) to anyone else for at least 3 months and during that time only do the things that you need/want to do for yourself or need/want to do at work or college or whatever. Also, pace yourself with the things that you need/want to do for yourself by prioritising what is the most pressing/urgent. Then break those tasks down into bite sized chunks, e.g. if something can be done over a few days, don't cram it into just one day. Also, always allow yourself some 'down time'/'me time' on a daily basis from all the activity to rest and relax. After the 3 months are up you should have a clearer idea of how much time you really have for offering to do things for other people; it may be that at present your life is too busy to spend a lot of time doing things for others, but you may realise that in a few months' time or whatever you will be able to spare some time to help them and know how much of your time you can offer...And when you do go back to offering your help make sure to tell those people you're helping that it may take a few days/weeks or whatever to complete whatever it is you're offering to do for them; that way you shouldn't feel under a lot of pressure to get things done for them asap. It sounds to me that you are a 'giver' and not a 'taker', so if and when people offer to help you with anything, consider accepting their offers of help. Look after yourself and protect yourself from 'burn out'.
All good wishes.
BloodMoon