If you are really interested in designing field then why did you go with execution field job? Can't you wait till the job come which you like? Is money matters? While doing work physical strain always is there. In the process of knowing the facts you want take u-turn? If you really doesn't want to work in that field, leave that job but not in the case of having physical strain, study and all.
If you come, now, decide. What you want to do exactly? What is your short-term and long-term goal? If money matters then you can earn money along with studies. Yes, this is possible. Some colleges will appointment you as a tech-quip and you can do M.Tec h in the same college. Search that type of college and join in that. But realize the fact that always stress will be there in the job (either mentally or physically). If you join in the college like tech-quip then you should have the capability to do both (study and work). Be preparing for that. I strongly suggest you to practice yoga or meditation daily for some time as it reduces the strain and ill-health. Now, the decision is in your hands.
Hope you will get success!